Re: [edk2-devel] [PATCH v4 3/5] OvmfPkg/PlatformInitLib: Add PlatformAddHobCB

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

On 1/25/23 03:11, Gerd Hoffmann wrote:
> On Tue, Jan 24, 2023 at 04:33:48PM -0600, Tom Lendacky wrote:
>> On 1/17/23 06:16, Gerd Hoffmann via groups.io wrote:
>>> Add PlatformAddHobCB() callback function for use with
>>> PlatformScanE820().  It adds HOBs for high memory and reservations (low
>>> memory is handled elsewhere because there are some special cases to
>>> consider).  This replaces calls to PlatformScanOrAdd64BitE820Ram() with
>>> AddHighHobs = TRUE.
>>>
>>> Write any actions done (adding HOBs, skip unknown types) to the firmware
>>> log with INFO loglevel.
>>>
>>> Also remove PlatformScanOrAdd64BitE820Ram() which is not used any more.
>>
>> Hi Gerd,
>>
>> A problem was reported to me for an SEV-ES guest that I bisected to this
>> patch. It only occurs when using the OVMF_CODE.fd file without specifying
>> the OVMF_VARS.fd file (i.e. only the one pflash device on the qemu command
>> line, but not using the OVMF.fd file). I don't ever boot without an
>> OVMF_VARS.fd file, so I didn't catch this.
>>
>> With this patch, SEV-ES terminates now because it detects doing MMIO to
>> encrypted memory area at 0xFFC00000 (where the OVMF_VARS.fd file would
>> normally be mapped). Prior to this commit, an SEV-ES guest booted without
>> issue in this configuration.
>>
>> First, is not specifying an OVMF_VARS.fd a valid configuration for booting
>> given the CODE/VARS split build?
> 
> No.

Ok, good to know.

> 
>> If it is valid, is the lack of the OVMF_VARS.fd resulting in the 0xFFC00000
>> address range getting marked reserved now (and thus mapped encrypted)?
> 
> I have no clue offhand.  The patch is not supposed to change OVMF
> behavior.  Adding the HOBs was done by the (increasingly messy)
> PlatformScanOrAdd64BitE820Ram() function before, with this patch in
> place PlatformScanE820() + PlatformAddHobCB() handle it instead.  The
> end result should be identical though.
> 
> OVMF does MMIO access @ 0xFFC00000, to check whenever it finds flash
> there or not (to handle the -bios OVMF.fd case).  That happens at a
> completely different place though (see
> OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c).
> 
>> Let me know if you need me to provide any output or testing if you can't
>> boot an SEV-ES guest.
> 
> Yes, the firmware log hopefully gives clues what is going on here.

So here are the differences (with some debug message that I added) between
booting at:

124b76505133 ("OvmfPkg/PlatformInitLib: Add PlatformGetLowMemoryCB")

   PlatformScanOrAdd64BitE820Ram: Reserved: Base=0xFEFFC000 Length=0x4000
   ...
   *** DEBUG: AmdSevDxeEntryPoint:120 - Clearing encryption bit for FF000000 to FFFFFFFF - MMIO=0
   *** DEBUG: AmdSevDxeEntryPoint:120 - Clearing encryption bit for 180000000 to 7FFFFFFFFFFF - MMIO=0
   ...
   QEMU Flash: Failed to find probe location
   QEMU flash was not detected. Writable FVB is not being installed.

and

328076cfdf45 ("OvmfPkg/PlatformInitLib: Add PlatformAddHobCB")

   PlatformAddHobCB: Reserved [0xFEFFC000, 0xFF000000)
   PlatformAddHobCB: HighMemory [0x100000000, 0x180000000)
   ...
   *** DEBUG: AmdSevDxeEntryPoint:120 - Clearing encryption bit for 1FDFFC000 to 7FFFFFFFFFFF - MMIO=0
   ...
   MMIO using encrypted memory: FFC00000
   !!!! X64 Exception Type - 0D(#GP - General Protection)  CPU Apic ID - 00000000 !!!!


So before the patch in question, we see that AmdSevDxeEntryPoint() in
OvmfPkg/AmdSevDxe/AmdSevDxe.c found an entry in the GCD map for 0xFF000000
to 0xFFFFFFFF that was marked as EfiGcdMemoryTypeNonExistent and so the
mapping was changed to unencrypted. But after that patch, that entry is
not present and so the 0xFFC00000 address is mapped encrypted and results
in the failure.

Thanks,
Tom

> 
> thanks,
>    Gerd
>