Re: [PATCH v2] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Use physical address with SEV-ES

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

On 1/23/21 7:57 AM, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3183
> 
> Under SEV-ES, a write to the flash device is done using a direct VMGEXIT
> to perform an MMIO write. The address provided to the MMIO write must be
> the physical address of the MMIO write destitnation. During boot, OVMF
> runs with an identity mapped pagetable structure so that VA == PA and the
> VMGEXIT MMIO write destination is just the virtual address of the flash
> area address being written.
> 
> However, when the UEFI SetVitualAddressMap() API is invoked, an identity
> mapped pagetable structure may not be in place and using the virtual
> address for the flash area address is no longer valid. This results in
> writes to the flash not being performed successfully. This can be seen
> by attempting to change the boot order under Linux. The update will
> appear to be performed, based on the output of the command. But rebooting
> the guest will show that the new boot order has not been set.
> 
> To remedy this, save the value of the flash base physical address before
> converting the address as part of SetVirtualAddressMap(). The physical
> address can then be calculated by obtaining the offset of the MMIO target
> virtual address relative to the flash base virtual address and adding that
> to the original flash base physical address. The resulting value produces
> a successful MMIO write during runtime services.
> 
> Fixes: 437eb3f7a8db7681afe0e6064d3a8edb12abb766
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> > ---
>   .../QemuFlashDxe.c                            | 20 ++++++++++++++++++-
>   1 file changed, 19 insertions(+), 1 deletion(-)

Forgot to add the changelog and use the diffstat options.

The changes address the comments from Laszlo on the v1 version of the patch:

- Update the last paragraph of the commit message
- Limit changes to only the QemuFlashDxe.c file
   - Update expression for calculating the physical address

Thanks,
Tom

> 
> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
> index 1b0742967f71..d303b0078b08 100644
> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
> @@ -16,11 +16,17 @@
>   
>   #include "QemuFlash.h"
>   
> +STATIC EFI_PHYSICAL_ADDRESS mSevEsFlashPhysBase;
> +
>   VOID
>   QemuFlashConvertPointers (
>     VOID
>     )
>   {
> +  if (MemEncryptSevEsIsEnabled ()) {
> +    mSevEsFlashPhysBase = (UINTN) mFlashBase;
> +  }
> +
>     EfiConvertPointer (0x0, (VOID **) &mFlashBase);
>   }
>   
> @@ -52,11 +58,23 @@ QemuFlashPtrWrite (
>     if (MemEncryptSevEsIsEnabled ()) {
>       MSR_SEV_ES_GHCB_REGISTER  Msr;
>       GHCB                      *Ghcb;
> +    EFI_PHYSICAL_ADDRESS      PhysAddr;
>       BOOLEAN                   InterruptState;
>   
>       Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
>       Ghcb = Msr.Ghcb;
>   
> +    //
> +    // The MMIO write needs to be to the physical address of the flash pointer.
> +    // Since this service is available as part of the EFI runtime services,
> +    // account for a non-identity mapped VA after SetVitualAddressMap().
> +    //
> +    if (mSevEsFlashPhysBase == 0) {
> +      PhysAddr = (UINTN) Ptr;
> +    } else {
> +      PhysAddr = mSevEsFlashPhysBase + (Ptr - mFlashBase);
> +    }
> +
>       //
>       // Writing to flash is emulated by the hypervisor through the use of write
>       // protection. This won't work for an SEV-ES guest because the write won't
> @@ -68,7 +86,7 @@ QemuFlashPtrWrite (
>       Ghcb->SharedBuffer[0] = Value;
>       Ghcb->SaveArea.SwScratch = (UINT64) (UINTN) Ghcb->SharedBuffer;
>       VmgSetOffsetValid (Ghcb, GhcbSwScratch);
> -    VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, (UINT64) (UINTN) Ptr, 1);
> +    VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, PhysAddr, 1);
>       VmgDone (Ghcb, InterruptState);
>     } else {
>       *Ptr = Value;
>