Re: [RFC PATCH v3 28/43] OvmfPkg: Create GHCB pages for use during Pei and Dxe phase

["Ni, Ray" <ray.ni@intel.com>]

> +  // Allocate GHCB and per-CPU variable pages.
> +  //
> +  GhcbPageCount = mMaxCpuCount * 2;
> +  GhcbBase = AllocatePages (GhcbPageCount);
> +  ASSERT (GhcbBase != NULL);
> +
> +  GhcbBasePa = (PHYSICAL_ADDRESS)(UINTN) GhcbBase;
> +
> +  DecryptStatus = MemEncryptSevClearPageEncMask (
> +    0,
> +    GhcbBasePa,
> +    GhcbPageCount,
> +    TRUE
> +    );
> +  ASSERT_RETURN_ERROR (DecryptStatus);
> +
> +  ZeroMem (GhcbBase, EFI_PAGES_TO_SIZE (GhcbPageCount));
> +
> +  PcdStatus = PcdSet64S (PcdGhcbBase, GhcbBasePa);
> +  ASSERT_RETURN_ERROR (PcdStatus);
> +  PcdStatus = PcdSet64S (PcdGhcbSize, EFI_PAGES_TO_SIZE (GhcbPageCount));
> +  ASSERT_RETURN_ERROR (PcdStatus);
> +
> +  DEBUG ((DEBUG_INFO,
> +    "SEV-ES is enabled, %lu GHCB pages allocated starting at 0x%p\n",
> +    (UINT64)GhcbPageCount, GhcbBase));
> +
> +  AsmWriteMsr64 (MSR_SEV_ES_GHCB, GhcbBasePa);

As I said in the comments to PcdGhcbBase/Size, can all PCD consumers read the GHCB MSR instead?

Does the GHCB buffer contains size information? If no, how does CPU know the GHCB buffer size?
I am asking this because I want to see a way to remove the PcdGhcbSize.

Thanks,
Ray