Re: [PATCH v4 2/4] OvmfPkg/BaseMemEncryptLib: Support to issue unencrypted hypercall

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 6/21/21 8:57 AM, Ashish Kalra wrote:
> From: Brijesh Singh <brijesh.singh@amd.com>
> 
> By default all the SEV guest memory regions are considered encrypted,
> if a guest changes the encryption attribute of the page (e.g mark a
> page as decrypted) then notify hypervisor. Hypervisor will need to
> track the unencrypted pages. The information will be used during
> guest live migration, guest page migration and guest debugging.
> 
> Invoke hypercall via the new hypercall library.
> 
> This hypercall is used to notify hypervisor when a page is marked as
> 'decrypted' (i.e C-bit removed).
> 
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>  OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf   |  1 +
>  OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf   |  1 +
>  OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c | 22 ++++++++++++++++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf b/OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf
> index f2e162d680..aefcd7c0f7 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf
> @@ -49,6 +49,7 @@
>    DebugLib
>    MemoryAllocationLib
>    PcdLib
> +  MemEncryptHypercallLib
>  
>  [FeaturePcd]
>    gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf b/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf
> index 03a78c32df..7503f56a0b 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf
> @@ -49,6 +49,7 @@
>    DebugLib
>    MemoryAllocationLib
>    PcdLib
> +  MemEncryptHypercallLib
>  
>  [FeaturePcd]
>    gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c
> index c696745f9d..12b3a9fcfb 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c
> @@ -15,6 +15,7 @@
>  #include <Library/MemEncryptSevLib.h>
>  #include <Register/Amd/Cpuid.h>
>  #include <Register/Cpuid.h>
> +#include <Library/MemEncryptHypercallLib.h>
>  
>  #include "VirtualMemory.h"
>  
> @@ -585,6 +586,9 @@ SetMemoryEncDec (
>    UINT64                         AddressEncMask;
>    BOOLEAN                        IsWpEnabled;
>    RETURN_STATUS                  Status;
> +  UINTN                          Size;
> +  BOOLEAN                        CBitChanged;
> +  PHYSICAL_ADDRESS               OrigPhysicalAddress;
>  
>    //
>    // Set PageMapLevel4Entry to suppress incorrect compiler/analyzer warnings.
> @@ -636,6 +640,10 @@ SetMemoryEncDec (
>  
>    Status = EFI_SUCCESS;
>  
> +  Size = Length;
> +  CBitChanged = FALSE;
> +  OrigPhysicalAddress = PhysicalAddress;
> +
>    while (Length != 0)
>    {
>      //
> @@ -695,6 +703,7 @@ SetMemoryEncDec (
>            ));
>          PhysicalAddress += BIT30;
>          Length -= BIT30;
> +        CBitChanged = TRUE;
>        } else {
>          //
>          // We must split the page
> @@ -749,6 +758,7 @@ SetMemoryEncDec (
>            SetOrClearCBit (&PageDirectory2MEntry->Uint64, Mode);
>            PhysicalAddress += BIT21;
>            Length -= BIT21;
> +          CBitChanged = TRUE;
>          } else {
>            //
>            // We must split up this page into 4K pages
> @@ -791,6 +801,7 @@ SetMemoryEncDec (
>          SetOrClearCBit (&PageTableEntry->Uint64, Mode);
>          PhysicalAddress += EFI_PAGE_SIZE;
>          Length -= EFI_PAGE_SIZE;
> +        CBitChanged = TRUE;
>        }
>      }
>    }
> @@ -808,6 +819,17 @@ SetMemoryEncDec (
>    //
>    CpuFlushTlb();
>  
> +  //
> +  // Notify Hypervisor on C-bit status
> +  //
> +  if (CBitChanged) {
> +    SetMemoryEncDecHypercall3 (
> +      OrigPhysicalAddress,
> +      EFI_SIZE_TO_PAGES(Size),
> +      KVM_MAP_GPA_RANGE_ENC_STAT(!Mode)
> +      );
> +  }

Maybe add a comment here that the "Mode" doesn't care whether the change
was for 4k vs 2mb vs 1gb. It is just tracking the total number of pages
changed (should you just remove those #defines for the page sizes then?)
which can be done by calculating everything at the 4k level.

Thanks,
Tom

> +
>  Done:
>    //
>    // Restore page table write protection, if any.
>