Re: [PATCH v14 06/22] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

[Chao Gao <chao.gao@intel.com>]

On Wed, Sep 10, 2025 at 09:46:01PM +0800, Xiaoyao Li wrote:
>On 9/10/2025 7:18 PM, Chao Gao wrote:
>> On Wed, Sep 10, 2025 at 05:37:50PM +0800, Xiaoyao Li wrote:
>> > On 9/9/2025 5:39 PM, Chao Gao wrote:
>> > > From: Sean Christopherson <seanjc@google.com>
>> > > 
>> > > Load the guest's FPU state if userspace is accessing MSRs whose values
>> > > are managed by XSAVES. Introduce two helpers, kvm_{get,set}_xstate_msr(),
>> > > to facilitate access to such kind of MSRs.
>> > > 
>> > > If MSRs supported in kvm_caps.supported_xss are passed through to guest,
>> > > the guest MSRs are swapped with host's before vCPU exits to userspace and
>> > > after it reenters kernel before next VM-entry.
>> > > 
>> > > Because the modified code is also used for the KVM_GET_MSRS device ioctl(),
>> > > explicitly check @vcpu is non-null before attempting to load guest state.
>> > > The XSAVE-managed MSRs cannot be retrieved via the device ioctl() without
>> > > loading guest FPU state (which doesn't exist).
>> > > 
>> > > 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.
>> 
>> ...
>> 
>> > > +	bool fpu_loaded = false;
>> > >    	int i;
>> > > -	for (i = 0; i < msrs->nmsrs; ++i)
>> > > +	for (i = 0; i < msrs->nmsrs; ++i) {
>> > > +		/*
>> > > +		 * If userspace is accessing one or more XSTATE-managed MSRs,
>> > > +		 * temporarily load the guest's FPU state so that the guest's
>> > > +		 * MSR value(s) is resident in hardware, i.e. so that KVM can
>> > > +		 * get/set the MSR via RDMSR/WRMSR.
>> > > +		 */
>> > > +		if (vcpu && !fpu_loaded && kvm_caps.supported_xss &&
>> > 
>> > why not check vcpu->arch.guest_supported_xss?
>> 
>> Looks like Sean anticipated someone would ask this question.
>
>here it determines whether to call kvm_load_guest_fpu().
>
>- based on kvm_caps.supported_xss, it will always load guest fpu.
>- based on vcpu->arch.guest_supported_xss, it depends on whether userspace
>calls KVM_SET_CPUID2 and whether it enables any XSS feature.
>
>So the difference is when no XSS feature is enabled for the VM.
>
>In this case, if checking vcpu->arch.guest_supported_xss, it will skip
>kvm_load_guest_fpu(). And it will result in GET_MSR gets usrerspace's value
>and SET_MSR changes userspace's value, when MSR access is eventually allowed
>in later do_msr() callback. Is my understanding correctly?

Actually, there will be no functional issue.

Those MSR accesses are always "rejected" with KVM_MSR_RET_UNSUPPORTED by
__kvm_set/get_msr() and get fixup if they are "host_initiated" in
kvm_do_msr_access(). KVM doesn't access any hardware MSRs in the process.

Using vcpu->arch.guest_supported_xss here also works, but the correctness
isn't that obvious for this special case.