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

[Xiaoyao Li <xiaoyao.li@intel.com>]

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?