Re: [PATCH v2 08/15] KVM: x86: Add Arch LBR MSRs to msrs_to_save_all list

[Sean Christopherson]

On Thu, Nov 24, 2022, Yang Weijiang wrote:
> Arch LBR MSR_ARCH_LBR_DEPTH and MSR_ARCH_LBR_CTL are queried by
> userspace application before it wants to {save|restore} the Arch LBR
> data. Other LBR related data MSRs are omitted here intentionally due
> to lengthy list(32*3). Userspace can still use KVM_{GET|SET}_MSRS to
> access them if necessary.

Absolutely not.  "there are a lot of them" isn't sufficient justification.  If
the MSRs need to be migrated, then KVM needs to report them.  If the expectation
is that XSAVES will handle them, then KVM needs to take a dependency on XSAVES
when enabling arch LBRs.