Re: [PATCH] KVM: x86: Allow XSAVES on CPUs where host doesn't use it due to an errata

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 23, 2023, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> Since commit b0563468eeac ("x86/CPU/AMD: Disable XSAVES on AMD family 0x17")
> kernel unconditionally clears the XSAVES CPU feature bit on Zen1/2 CPUs.
> 
> Since KVM CPU caps are initialized from the kernel boot CPU features this
> makes the XSAVES feature also unavailable for KVM guests in this case, even
> though they might want to decide on their own whether they are affected by
> this errata.
> 
> Allow KVM guests to make such decision by setting the XSAVES KVM CPU
> capability bit based on the actual CPU capability

This is not generally safe, as the guest can make such a decision if and only if
the Family/Model/Stepping information is reasonably accurate.

> This fixes booting Hyper-V enabled Windows Server 2016 VMs with more than
> one vCPU on Zen1/2 CPUs.

How/why does lack of XSAVES break a multi-vCPU setup?  Is Windows blindly doing
XSAVES based on FMS?