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

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

On 1/25/21 4:17 PM, Laszlo Ersek wrote:
> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.tianocore.org%2Fshow_bug.cgi%3Fid%3D3183&amp;data=04%7C01%7Cthomas.lendacky%40amd.com%7Ce7ba4f968a0d4bc42ea108d8c17f0a9b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637472098694077769%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=jGAzh8XsefGmSnQfjjz%2BQmLJoUu25o67pOxuUORzw7g%3D&amp;reserved=0
>>>
>>> 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.)

Well, looks like I was only able to get it right once...  I just noticed
down in the code comment it is wrong, too. Ugh, my fingers just couldn't
type that 'r' I guess.

Thanks,
Tom

> 
> 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 address for the flash area address is no longer valid.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;
>>>
>>
>