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SubjectRe: [PATCH 0/7] KVM: x86: guest MAXPHYADDR and C-bit fixes
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On 6/24/21 12:31 PM, Sean Christopherson wrote:
> On Thu, Jun 24, 2021, Tom Lendacky wrote:
>>>
>>> Here's an explanation of the physical address reduction for bare-metal and
>>> guest.
>>>
>>> With MSR 0xC001_0010[SMEE] = 0:
>>> No reduction in host or guest max physical address.
>>>
>>> With MSR 0xC001_0010[SMEE] = 1:
>>> - Reduction in the host is enumerated by CPUID 0x8000_001F_EBX[11:6],
>>> regardless of whether SME is enabled in the host or not. So, for example
>>> on EPYC generation 2 (Rome) you would see a reduction from 48 to 43.
>>> - There is no reduction in physical address in a legacy guest (non-SEV
>>> guest), so the guest can use a 48-bit physical address
>
> So the behavior I'm seeing is either a CPU bug or user error. Can you verify
> the unexpected #PF behavior to make sure I'm not doing something stupid?

Yeah, I saw that in patch #3. Let me see what I can find out. I could just
be wrong on that myself - it wouldn't be the first time.

Thanks,
Tom

>
> Thanks!
>
>>> - There is a reduction of only the encryption bit in an SEV guest, so
>>> the guest can use up to a 47-bit physical address. This is why the
>>> Qemu command line sev-guest option uses a value of 1 for the
>>> "reduced-phys-bits" parameter.
>>>
>>
>> The guest statements all assume that NPT is enabled.
>>
>> Thanks,
>> Tom

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