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

["Laszlo Ersek" <lersek@redhat.com>]

On 01/25/21 14:55, Tom Lendacky wrote:
> 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
> 
> s/SetVitualAddressMap/SetVirtualAddressMap/
> 
> I can fix that if another version is required, otherwise, can it be fixed
> on commit?

Yes, I'll fix it on merge. I made the exact same typo in my v1 comment
-- and that was because I copied the mistyped string from the BZ :) (See
c#0, c#1.)

Thanks!
Laszlo

> 
> Thanks,
> Tom
> 
>> 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 address for the flash area address is no longer valid.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(-)
>>
>> 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;
>>
>