Re: [RFC PATCH 12/19] OvmfPkg/MemEncryptSevLib: Add support to validate system RAM

["Brijesh Singh" <brijesh.singh@amd.com>]

Hi Yao,

On 4/1/21 1:37 AM, Yao, Jiewen wrote:
> Hi
> Would you please clarify why the IA32 version implementation is empty?
>
> Does it mean IA32 does not need validate?
> Or IA32 should never call this function?
>
> Anyway, I recommend to add some comment to describe it clearly.
> If it should never be called, I recommend to add ASSERT(FALSE).


AMD SEV, SEV-ES and SEV-SNP support is not available for the IA32
architecture. Nobody should be calling it. I will add an ASSERT().

Thanks

>
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/Ia32/SnpPageStateChange.c
> @@ -0,0 +1,17 @@
> +#include <Uefi/UefiBaseType.h>
> +
> +#include "../SnpPageStateChange.h"
> +
> +/**
> + The function is used to set the page state when SEV-SNP is active. The page state
> + transition consist of changing the page ownership in the RMP table, and using the
> + PVALIDATE instruction to update the Validated bit in RMP table.
> +
> + */
> +VOID
> +SevSnpValidateSystemRamInternal (
> +  IN EFI_PHYSICAL_ADDRESS             BaseAddress,
> +  IN UINTN                            NumPages
> +  )
> +{
> +}
>
>> -----Original Message-----
>> From: Brijesh Singh <brijesh.singh@amd.com>
>> Sent: Wednesday, March 24, 2021 11:32 PM
>> To: devel@edk2.groups.io
>> Cc: Brijesh Singh <brijesh.singh@amd.com>; James Bottomley
>> <jejb@linux.ibm.com>; Xu, Min M <min.m.xu@intel.com>; Yao, Jiewen
>> <jiewen.yao@intel.com>; Tom Lendacky <thomas.lendacky@amd.com>; Justen,
>> Jordan L <jordan.l.justen@intel.com>; Ard Biesheuvel
>> <ardb+tianocore@kernel.org>; Laszlo Ersek <lersek@redhat.com>
>> Subject: [RFC PATCH 12/19] OvmfPkg/MemEncryptSevLib: Add support to
>> validate system RAM
>>
>> BZ: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.tianocore.org%2Fshow_bug.cgi%3Fid%3D3275&amp;data=04%7C01%7Cbrijesh.singh%40amd.com%7C599bf9c821c94c0241ed08d8f4d8b2a5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637528558846316719%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=4Lq97rE69lPFAsGjYb44e3u2mA9QlrJdayxTP9FG7w8%3D&amp;reserved=0
>>
>> Many of the integrity guarantees of SEV-SNP are enforced through the
>> Reverse Map Table (RMP). Each RMP entry contains the GPA at which a
>> particular page of DRAM should be mapped. The guest can request the
>> hypervisor to add pages in the RMP table via the Page State Change VMGEXIT
>> defined in the GHCB specification section 2.5.1 and 4.1.6. Inside each RMP
>> entry is a Validated flag; this flag is automatically cleared to 0 by the
>> CPU hardware when a new RMP entry is created for a guest. Each VM page
>> can be either validated or invalidated, as indicated by the Validated
>> flag in the RMP entry. Memory access to a private page that is not
>> validated generates a #VC. A VM can use the PVALIDATE instruction to
>> validate the private page before using it.
>>
>> During the guest creation, the boot ROM memory is pre-validated by the
>> AMD-SEV firmware. The MemEncryptSevSnpValidateSystemRam() can be called
>> during the SEC and PEI phase to validate the detected system RAM.
>>
>> One of the fields in the Page State Change NAE is the RMP page size. The
>> page size input parameter indicates that either a 4KB or 2MB page should
>> be used while adding the RMP entry. During the validation, when possible,
>> the MemEncryptSevSnpValidateSystemRam() will use the 2MB entry. A
>> hypervisor backing the memory may choose to use the different page size
>> in the RMP entry. In those cases, the PVALIDATE instruction should return
>> SIZEMISMATCH. If a SIZEMISMATCH is detected, then validate all 512-pages
>> constituting a 2MB region.
>>
>> Upon completion, the PVALIDATE instruction sets the rFLAGS.CF to 0 if
>> instruction changed the RMP entry and to 1 if the instruction did not
>> change the RMP entry. The rFlags.CF will be 1 only when a memory region
>> is already validated. We should not double validate a memory
>> as it could lead to a security compromise. If double validation is
>> detected, terminate the boot.
>>
>> Cc: James Bottomley <jejb@linux.ibm.com>
>> Cc: Min Xu <min.m.xu@intel.com>
>> Cc: Jiewen Yao <jiewen.yao@intel.com>
>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
>> Cc: Laszlo Ersek <lersek@redhat.com>
>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>> ---
>>  OvmfPkg/Include/Library/MemEncryptSevLib.h                            |  15 ++
>>  OvmfPkg/Library/BaseMemEncryptSevLib/Ia32/SnpPageStateChange.c        |  17
>> ++
>>  OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf          |   4 +
>>  OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLibInternal.c    |
>> 20 ++
>>  OvmfPkg/Library/BaseMemEncryptSevLib/SnpPageStateChange.h             |  37
>> +++
>>  OvmfPkg/Library/BaseMemEncryptSevLib/X64/SecSnpSystemRamValidate.c    |
>> 23 ++
>>  OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c |
>> 254 ++++++++++++++++++++
>>  7 files changed, 370 insertions(+)
>>
>> diff --git a/OvmfPkg/Include/Library/MemEncryptSevLib.h
>> b/OvmfPkg/Include/Library/MemEncryptSevLib.h
>> index 03d9eda392..47d6802b61 100644
>> --- a/OvmfPkg/Include/Library/MemEncryptSevLib.h
>> +++ b/OvmfPkg/Include/Library/MemEncryptSevLib.h
>> @@ -215,4 +215,19 @@ MemEncryptSevGetAddressRangeState (
>>    IN UINTN                    Length
>>    );
>>
>> +/**
>> +  If SEV-SNP is active then set the page state of the specified virtual
>> +  address range. This should be called in SEC and PEI phases only.
>> +
>> +  @param[in]  BaseAddress             Base address
>> +  @param[in]  NumPages                Number of pages starting from the base
>> address
>> +
>> +**/
>> +VOID
>> +EFIAPI
>> +MemEncryptSevSnpValidateSystemRam (
>> +  IN PHYSICAL_ADDRESS                   BaseAddress,
>> +  IN UINTN                              NumPages
>> +  );
>> +
>>  #endif // _MEM_ENCRYPT_SEV_LIB_H_
>> diff --git
>> a/OvmfPkg/Library/BaseMemEncryptSevLib/Ia32/SnpPageStateChange.c
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/Ia32/SnpPageStateChange.c
>> new file mode 100644
>> index 0000000000..dace5c0bcf
>> --- /dev/null
>> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/Ia32/SnpPageStateChange.c
>> @@ -0,0 +1,17 @@
>> +#include <Uefi/UefiBaseType.h>
>> +
>> +#include "../SnpPageStateChange.h"
>> +
>> +/**
>> + The function is used to set the page state when SEV-SNP is active. The page
>> state
>> + transition consist of changing the page ownership in the RMP table, and using
>> the
>> + PVALIDATE instruction to update the Validated bit in RMP table.
>> +
>> + */
>> +VOID
>> +SevSnpValidateSystemRamInternal (
>> +  IN EFI_PHYSICAL_ADDRESS             BaseAddress,
>> +  IN UINTN                            NumPages
>> +  )
>> +{
>> +}
>> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf
>> index 279c38bfbc..8595e244c2 100644
>> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf
>> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf
>> @@ -31,15 +31,19 @@
>>
>>  [Sources]
>>    SecMemEncryptSevLibInternal.c
>> +  SnpPageStateChange.h
>>
>>  [Sources.X64]
>>    X64/MemEncryptSevLib.c
>>    X64/SecVirtualMemory.c
>> +  X64/SecSnpSystemRamValidate.c
>> +  X64/SnpPageStateChangeInternal.c
>>    X64/VirtualMemory.c
>>    X64/VirtualMemory.h
>>
>>  [Sources.IA32]
>>    Ia32/MemEncryptSevLib.c
>> +  Ia32/SnpPageStateChange.c
>>
>>  [LibraryClasses]
>>    BaseLib
>> diff --git
>> a/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLibInternal.c
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLibInternal.c
>> index 69852779e2..35a222e75e 100644
>> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLibInternal.c
>> +++
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLibInternal.c
>> @@ -17,6 +17,8 @@
>>  #include <Register/Cpuid.h>
>>  #include <Uefi/UefiBaseType.h>
>>
>> +#include "SnpPageStateChange.h"
>> +
>>  /**
>>    Reads and sets the status of SEV features.
>>
>> @@ -172,3 +174,21 @@
>> MemEncryptSevLocateInitialSmramSaveStateMapPages (
>>  {
>>    return RETURN_UNSUPPORTED;
>>  }
>> +
>> +/**
>> +  If SEV-SNP is active then set the page state of the specified virtual
>> +  address range. This should be called in SEC and PEI phases only.
>> +
>> +  @param[in]  BaseAddress             Base address
>> +  @param[in]  NumPages                Number of pages starting from the base
>> address
>> +
>> +**/
>> +VOID
>> +EFIAPI
>> +MemEncryptSevSnpValidateSystemRam (
>> +  IN PHYSICAL_ADDRESS                   BaseAddress,
>> +  IN UINTN                              NumPages
>> +  )
>> +{
>> +  SevSnpValidateSystemRam (BaseAddress, NumPages);
>> +}
>> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/SnpPageStateChange.h
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/SnpPageStateChange.h
>> new file mode 100644
>> index 0000000000..3040930999
>> --- /dev/null
>> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/SnpPageStateChange.h
>> @@ -0,0 +1,37 @@
>> +/** @file
>> +
>> +  SEV-SNP Page Validation functions.
>> +
>> +  Copyright (c) 2020 - 2021, AMD Incorporated. All rights reserved.<BR>
>> +
>> +  SPDX-License-Identifier: BSD-2-Clause-Patent
>> +
>> +**/
>> +
>> +#ifndef SNP_PAGE_STATE_INTERNAL_H_
>> +#define SNP_PAGE_STATE_INTERNAL_H_
>> +
>> +//
>> +// SEV-SNP Page states
>> +//
>> +typedef enum {
>> +  SevSnpPagePrivate,
>> +  SevSnpPageShared,
>> +
>> +} SEV_SNP_PAGE_STATE;
>> +
>> +VOID
>> +SevSnpValidateSystemRam (
>> +  IN EFI_PHYSICAL_ADDRESS             BaseAddress,
>> +  IN UINTN                            NumPages
>> +  );
>> +
>> +VOID
>> +SetPageStateInternal (
>> +  IN EFI_PHYSICAL_ADDRESS             BaseAddress,
>> +  IN UINTN                            NumPages,
>> +  IN SEV_SNP_PAGE_STATE               State,
>> +  IN BOOLEAN                          UseLargeEntry
>> +  );
>> +
>> +#endif
>> diff --git
>> a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SecSnpSystemRamValidate.c
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SecSnpSystemRamValidate.c
>> new file mode 100644
>> index 0000000000..915706aad0
>> --- /dev/null
>> +++
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SecSnpSystemRamValidate.c
>> @@ -0,0 +1,23 @@
>> +/** @file
>> +
>> +  SEV-SNP Page Validation functions.
>> +
>> +  Copyright (c) 2020 - 2021, AMD Incorporated. All rights reserved.<BR>
>> +
>> +  SPDX-License-Identifier: BSD-2-Clause-Patent
>> +
>> +**/
>> +
>> +#include <Uefi/UefiBaseType.h>
>> +#include <Library/BaseLib.h>
>> +
>> +#include "../SnpPageStateChange.h"
>> +
>> +VOID
>> +SevSnpValidateSystemRam (
>> +  IN EFI_PHYSICAL_ADDRESS             BaseAddress,
>> +  IN UINTN                            NumPages
>> +  )
>> +{
>> +  SetPageStateInternal (BaseAddress, NumPages, SevSnpPagePrivate, TRUE);
>> +}
>> diff --git
>> a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c
>> new file mode 100644
>> index 0000000000..5a34db33fe
>> --- /dev/null
>> +++
>> b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/SnpPageStateChangeInternal.c
>> @@ -0,0 +1,254 @@
>> +/** @file
>> +
>> +  SEV-SNP Page Validation functions.
>> +
>> +  Copyright (c) 2020 - 2021, AMD Incorporated. All rights reserved.<BR>
>> +
>> +  SPDX-License-Identifier: BSD-2-Clause-Patent
>> +
>> +**/
>> +
>> +#include <Uefi/UefiBaseType.h>
>> +#include <Library/BaseLib.h>
>> +#include <Library/BaseMemoryLib.h>
>> +#include <Library/MemEncryptSevLib.h>
>> +#include <Library/DebugLib.h>
>> +#include <Library/VmgExitLib.h>
>> +
>> +#include <Register/Amd/Ghcb.h>
>> +#include <Register/Amd/Msr.h>
>> +
>> +#include "../SnpPageStateChange.h"
>> +
>> +#define IS_ALIGNED(x, y)        ((((x) & (y - 1)) == 0))
>> +#define PAGES_PER_LARGE_ENTRY   512
>> +#define EFI_LARGE_PAGE          (EFI_PAGE_SIZE * PAGES_PER_LARGE_ENTRY)
>> +
>> +STATIC
>> +UINTN
>> +MemoryStateToGhcbOp (
>> +  IN SEV_SNP_PAGE_STATE   State
>> +  )
>> +{
>> +  UINTN Cmd;
>> +
>> +  switch (State) {
>> +    case SevSnpPageShared: Cmd = SNP_PAGE_STATE_SHARED; break;
>> +    case SevSnpPagePrivate: Cmd = SNP_PAGE_STATE_PRIVATE; break;
>> +    default: ASSERT(0);
>> +  }
>> +
>> +  return Cmd;
>> +}
>> +
>> +STATIC
>> +VOID
>> +SnpPageStateFailureTerminate (
>> +  VOID
>> +  )
>> +{
>> +  MSR_SEV_ES_GHCB_REGISTER  Msr;
>> +
>> +  //
>> +  // Use the GHCB MSR Protocol to request termination by the hypervisor
>> +  //
>> +  Msr.GhcbPhysicalAddress = 0;
>> +  Msr.GhcbTerminate.Function = GHCB_INFO_TERMINATE_REQUEST;
>> +  Msr.GhcbTerminate.ReasonCodeSet = GHCB_TERMINATE_GHCB;
>> +  Msr.GhcbTerminate.ReasonCode = GHCB_TERMINATE_GHCB_GENERAL;
>> +  AsmWriteMsr64 (MSR_SEV_ES_GHCB, Msr.GhcbPhysicalAddress);
>> +
>> +  AsmVmgExit ();
>> +
>> +  ASSERT (FALSE);
>> +  CpuDeadLoop ();
>> +}
>> +
>> +STATIC
>> +UINTN
>> +IssuePvalidate (
>> +  IN    UINTN       Address,
>> +  IN    UINTN       RmpPageSize,
>> +  IN    BOOLEAN     Validate
>> +  )
>> +{
>> +  IA32_EFLAGS32         EFlags;
>> +  UINTN                 Ret;
>> +
>> +  Ret = AsmPvalidate (RmpPageSize, Validate, Address, &EFlags);
>> +
>> +  //
>> +  // Check the rFlags.CF to verify that PVALIDATE updated the RMP
>> +  // entry. If there was a no change in the RMP entry then we are
>> +  // either double validating or invalidating the memory. This can
>> +  // lead to a security compromise.
>> +  //
>> +  if (EFlags.Bits.CF) {
>> +    DEBUG ((DEBUG_ERROR, "%a:%a: Double %a detected for address 0x%Lx\n",
>> +           gEfiCallerBaseName,
>> +           __FUNCTION__,
>> +           Validate ? "Validate" : "Invalidate",
>> +           Address));
>> +    SnpPageStateFailureTerminate ();
>> +  }
>> +
>> +  return Ret;
>> +}
>> +
>> +/**
>> + This function issues the PVALIDATE instruction to validate or invalidate the
>> memory
>> + range specified. If PVALIDATE returns size mismatch then it tries validating
>> with
>> + smaller page size.
>> +
>> + */
>> +STATIC
>> +VOID
>> +PvalidateRange (
>> +  IN  SNP_PAGE_STATE_CHANGE_INFO    *Info,
>> +  IN  UINTN                         StartIndex,
>> +  IN  UINTN                         EndIndex,
>> +  IN  BOOLEAN                       Validate
>> +  )
>> +{
>> +  UINTN         Address, RmpPageSize, Ret, i;
>> +
>> +  for (; StartIndex < EndIndex; StartIndex++) {
>> +    Address = Info->Entry[StartIndex].GuestFrameNumber << EFI_PAGE_SHIFT;
>> +    RmpPageSize = Info->Entry[StartIndex].PageSize;
>> +
>> +    Ret = IssuePvalidate (Address, RmpPageSize, Validate);
>> +
>> +    //
>> +    // If we fail to validate due to size mismatch then try with the
>> +    // smaller page size. This senario will occur if the backing page in
>> +    // the RMP entry is 4K and we are validating it as a 2MB.
>> +    //
>> +    if ((Ret == PVALIDATE_RET_FAIL_SIZEMISMATCH) &&
>> +        (RmpPageSize == PVALIDATE_PAGE_SIZE_2M)) {
>> +      for (i = 0; i < PAGES_PER_LARGE_ENTRY; i++) {
>> +
>> +        Ret = IssuePvalidate (Address, PVALIDATE_PAGE_SIZE_4K, Validate);
>> +        if (Ret) {
>> +          break;
>> +        }
>> +
>> +        Address = Address + EFI_PAGE_SIZE;
>> +      }
>> +    }
>> +
>> +    if (Ret) {
>> +    DEBUG ((DEBUG_ERROR, "%a:%a: Failed to %a address 0x%Lx Error
>> code %d\n",
>> +           gEfiCallerBaseName,
>> +           __FUNCTION__,
>> +           Validate ? "Validate" : "Invalidate",
>> +           Address,
>> +           Ret));
>> +      SnpPageStateFailureTerminate ();
>> +    }
>> +  }
>> +}
>> +
>> +/**
>> + The function is used to set the page state when SEV-SNP is active. The page
>> state
>> + transition consist of changing the page ownership in the RMP table, and using
>> the
>> + PVALIDATE instruction to update the Validated bit in RMP table.
>> +
>> + When the UseLargeEntry is set to TRUE, then use the large RMP entry.
>> + */
>> +VOID
>> +SetPageStateInternal (
>> +  IN EFI_PHYSICAL_ADDRESS             BaseAddress,
>> +  IN UINTN                            NumPages,
>> +  IN SEV_SNP_PAGE_STATE               State,
>> +  IN BOOLEAN                          UseLargeEntry
>> +  )
>> +{
>> +  EFI_STATUS                      Status;
>> +  GHCB                            *Ghcb;
>> +  EFI_PHYSICAL_ADDRESS            NextAddress, EndAddress;
>> +  MSR_SEV_ES_GHCB_REGISTER        Msr;
>> +  BOOLEAN                         InterruptState;
>> +  SNP_PAGE_STATE_CHANGE_INFO      *Info;
>> +  UINTN                           i, RmpPageSize;
>> +
>> +  Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
>> +  Ghcb = Msr.Ghcb;
>> +
>> +  EndAddress = BaseAddress + EFI_PAGES_TO_SIZE (NumPages);
>> +
>> +  DEBUG ((DEBUG_VERBOSE, "%a:%a Address 0x%Lx - 0x%Lx State = %a
>> LargeEntry = %d\n",
>> +          gEfiCallerBaseName,
>> +          __FUNCTION__,
>> +          BaseAddress,
>> +          EndAddress,
>> +          State == SevSnpPageShared ? "Shared" : "Private",
>> +          UseLargeEntry));
>> +
>> +  for (; BaseAddress < EndAddress; BaseAddress = NextAddress) {
>> +
>> +    //
>> +    // Initialize the GHCB and setup scratch sw to point to shared buffer.
>> +    //
>> +    VmgInit (Ghcb, &InterruptState);
>> +    Info = (SNP_PAGE_STATE_CHANGE_INFO *) Ghcb->SharedBuffer;
>> +
>> +    SetMem (Info, sizeof (*Info), 0);
>> +
>> +    //
>> +    // Build page state change buffer
>> +    //
>> +    for (i = 0; (EndAddress > BaseAddress) && i < SNP_PAGE_STATE_MAX_ENTRY;
>> +          BaseAddress = NextAddress, i++) {
>> +      //
>> +      // Is this a 2MB aligned page? Check if we can use the Large RMP entry.
>> +      //
>> +      if (UseLargeEntry &&
>> +          IS_ALIGNED (BaseAddress, EFI_LARGE_PAGE) &&
>> +          ((EndAddress - BaseAddress) >> EFI_PAGE_SHIFT) >=
>> PAGES_PER_LARGE_ENTRY) {
>> +        RmpPageSize = PVALIDATE_PAGE_SIZE_2M;
>> +        NextAddress = BaseAddress + EFI_LARGE_PAGE;
>> +      } else {
>> +        RmpPageSize = PVALIDATE_PAGE_SIZE_4K;
>> +        NextAddress = BaseAddress + EFI_PAGE_SIZE;
>> +      }
>> +
>> +      Info->Entry[i].GuestFrameNumber = BaseAddress >> EFI_PAGE_SHIFT;
>> +      Info->Entry[i].PageSize = RmpPageSize;
>> +      Info->Entry[i].Op = MemoryStateToGhcbOp (State);
>> +      Info->Entry[i].CurrentPage = 0;
>> +    }
>> +
>> +    Info->Header.CurrentEntry = 0;
>> +    Info->Header.EndEntry = i - 1;
>> +
>> +    //
>> +    // If the request page state change is shared then invalidate the pages before
>> +    // adding the page in the RMP table.
>> +    //
>> +    if (State == SevSnpPageShared) {
>> +      PvalidateRange (Info, 0, i, FALSE);
>> +    }
>> +
>> +    //
>> +    // Issue the VMGEXIT and retry if hypervisor failed to process all the entries.
>> +    //
>> +    Ghcb->SaveArea.SwScratch = (UINT64) Ghcb->SharedBuffer;
>> +    VmgSetOffsetValid (Ghcb, GhcbSwScratch);
>> +    while (Info->Header.CurrentEntry <= Info->Header.EndEntry) {
>> +      Status = VmgExit (Ghcb, SVM_EXIT_SNP_PAGE_STATE_CHANGE, 0, 0);
>> +      if (EFI_ERROR (Status)) {
>> +        SnpPageStateFailureTerminate ();
>> +      }
>> +    }
>> +
>> +    //
>> +    // If the request page state change is shared then invalidate the pages before
>> +    // adding the page in the RMP table.
>> +    //
>> +    if (State == SevSnpPagePrivate) {
>> +      PvalidateRange (Info, 0, i, TRUE);
>> +    }
>> +
>> +    VmgDone (Ghcb, InterruptState);
>> +  }
>> +}
>> --
>> 2.17.1