Re: [PATCH v1 2/2] OvmfPkg/BaseMemEncryptLib: Support to issue unencrypted hypercall

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

On 12/2/20 1:29 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).

This will miss the SEC GHCB page that is mapped as unencrypted in 
OvmfPkg/ResetVector/Ia32/PageTables64.asm. You'll need to remember to mark 
that one specifically. I don't think there are any others.

Thanks,
Tom

> 
> 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/BaseMemEncryptSevLib.inf |  1 +
>   OvmfPkg/Library/BaseMemEncryptSevLib/X64/VirtualMemory.c      | 18 ++++++++++++++++++
>   2 files changed, 19 insertions(+)
> 
> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/BaseMemEncryptSevLib.inf b/OvmfPkg/Library/BaseMemEncryptSevLib/BaseMemEncryptSevLib.inf
> index 7c44d09528..95ee707918 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/BaseMemEncryptSevLib.inf
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/BaseMemEncryptSevLib.inf
> @@ -46,6 +46,7 @@
>     DebugLib
>     MemoryAllocationLib
>     PcdLib
> +  MemEncryptHypercallLib
>   
>   [FeaturePcd]
>     gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/VirtualMemory.c b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/VirtualMemory.c
> index 5e110c84ff..1e670b6200 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/VirtualMemory.c
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/VirtualMemory.c
> @@ -14,6 +14,7 @@
>   #include <Library/CpuLib.h>
>   #include <Register/Amd/Cpuid.h>
>   #include <Register/Cpuid.h>
> +#include <Library/MemEncryptHypercallLib.h>
>   
>   #include "VirtualMemory.h"
>   
> @@ -589,6 +590,9 @@ SetMemoryEncDec (
>     UINT64                         AddressEncMask;
>     BOOLEAN                        IsWpEnabled;
>     RETURN_STATUS                  Status;
> +  UINTN                          Size;
> +  BOOLEAN                        CBitChanged;
> +  PHYSICAL_ADDRESS               OrigPhysicalAddress;
>   
>     //
>     // Set PageMapLevel4Entry to suppress incorrect compiler/analyzer warnings.
> @@ -640,6 +644,10 @@ SetMemoryEncDec (
>   
>     Status = EFI_SUCCESS;
>   
> +  Size = Length;
> +  CBitChanged = FALSE;
> +  OrigPhysicalAddress = PhysicalAddress;
> +
>     while (Length)
>     {
>       //
> @@ -699,6 +707,7 @@ SetMemoryEncDec (
>             ));
>           PhysicalAddress += BIT30;
>           Length -= BIT30;
> +        CBitChanged = TRUE;
>         } else {
>           //
>           // We must split the page
> @@ -753,6 +762,7 @@ SetMemoryEncDec (
>             SetOrClearCBit (&PageDirectory2MEntry->Uint64, Mode);
>             PhysicalAddress += BIT21;
>             Length -= BIT21;
> +          CBitChanged = TRUE;
>           } else {
>             //
>             // We must split up this page into 4K pages
> @@ -795,6 +805,7 @@ SetMemoryEncDec (
>           SetOrClearCBit (&PageTableEntry->Uint64, Mode);
>           PhysicalAddress += EFI_PAGE_SIZE;
>           Length -= EFI_PAGE_SIZE;
> +        CBitChanged = TRUE;
>         }
>       }
>     }
> @@ -812,6 +823,13 @@ SetMemoryEncDec (
>     //
>     CpuFlushTlb();
>   
> +  //
> +  // Notify Hypervisor on C-bit status
> +  //
> +  if (CBitChanged) {
> +    SetMemoryEncDecHypercall3 (OrigPhysicalAddress, EFI_SIZE_TO_PAGES(Size), !Mode);
> +  }
> +
>   Done:
>     //
>     // Restore page table write protection, if any.
>