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@xxxxxxxxxx>
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?
