linux-kernel - Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch

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Message-ID: <ae5b0195-b04f-8eef-9e0d-2a46c761d2d5@redhat.com>
Date:   Mon, 8 Feb 2021 11:25:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jing Liu <jing2.liu@...ux.intel.com>, seanjc@...gle.com,
        kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, jing2.liu@...el.com
Subject: Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch

On 07/02/21 16:42, Jing Liu wrote:
> |In KVM, "guest_fpu" serves for any guest task working on this vcpu 
> during vmexit and vmenter. We provide a pre-allocated guest_fpu space 
> and entire "guest_fpu.state_mask" to avoid each dynamic features 
> detection on each vcpu task. Meanwhile, to ensure correctly 
> xsaves/xrstors guest state, set IA32_XFD as zero during vmexit and vmenter.|

Most guests will not need the whole xstate feature set.  So perhaps you 
  could set XFD to the host value | the guest value, trap #NM if the 
host XFD is zero, and possibly reflect the exception to the guest's XFD 
and XFD_ERR.

In addition, loading the guest XFD MSRs should use the MSR autoload 
feature (add_atomic_switch_msr).

Paolo