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

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Message-ID: <c2d7a69a-386e-6f44-71c2-eb9a243c3a78@amd.com>
Date:   Thu, 24 Jun 2021 11:36:37 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     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

> 
> 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
> - 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