linux-kernel - Re: [RESEND v13 06/11] KVM: x86: Load guest fpu state when access MSRs managed by XSAVES

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Message-ID: <20200722203244.GG9114@linux.intel.com>
Date:   Wed, 22 Jul 2020 13:32:45 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, jmattson@...gle.com,
        yu.c.zhang@...ux.intel.com
Subject: Re: [RESEND v13 06/11] KVM: x86: Load guest fpu state when access
 MSRs managed by XSAVES

On Thu, Jul 16, 2020 at 11:16:22AM +0800, Yang Weijiang wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> A handful of CET MSRs are not context switched through "traditional"
> methods, e.g. VMCS or manual switching, but rather are passed through
> to the guest and are saved and restored by XSAVES/XRSTORS, i.e. in the
> guest's FPU state.
> 
> Load the guest's FPU state if userspace is accessing MSRs whose values
> are managed by XSAVES so that the MSR helper, e.g. vmx_{get,set}_msr(),
> can simply do {RD,WR}MSR to access the guest's value.
> 
> Note that guest_cpuid_has() is not queried as host userspace is allowed
> to access MSRs that have not been exposed to the guest, e.g. it might do
> KVM_SET_MSRS prior to KVM_SET_CPUID2.

No comments on the patch itself.  Added a blurb to the changelog to call
out the vcpu==NULL case is possible due to KVM_GET_MSRS also being a device
scope ioctl().

> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Co-developed-by: Yang Weijiang <weijiang.yang@...el.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>