linux-kernel - Re: [PATCH 0/7] KVM: x86: guest MAXPHYADDR and C-bit fixes

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Date:   Thu, 24 Jun 2021 17:07:58 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Gonda <pgonda@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH 0/7] KVM: x86: guest MAXPHYADDR and C-bit fixes

On 6/24/21 12:39 PM, Tom Lendacky wrote:
> 
> 
> 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.

>From patch #3:
  SVM: KVM: CPU #PF @ rip = 0x409ca4, cr2 = 0xc0000000, pfec = 0xb
  KVM: guest PTE = 0x181023 @ GPA = 0x180000, level = 4
  KVM: guest PTE = 0x186023 @ GPA = 0x181000, level = 3
  KVM: guest PTE = 0x187023 @ GPA = 0x186000, level = 2
  KVM: guest PTE = 0xffffbffff003 @ GPA = 0x187000, level = 1
  SVM: KVM: GPA = 0x7fffbffff000

I think you may be hitting a special HT region that is at the top 12GB of
the 48-bit memory range and is reserved, even for GPAs. Can you somehow
get the test to use an address below 0xfffd_0000_0000? That would show
that bit 47 is valid for the legacy guest while staying out of the HT region.

Thanks,
Tom

> 
> 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