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

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

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.
> 
> The two helpers are put here in order to manifest accessing xsave-managed
> MSRs requires special check and handling to guarantee the correctness of
> read/write to the MSRs.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Co-developed-by: Yang Weijiang <weijiang.yang@intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
> v14:
> - s/rdmsrl/rdmsrq, s/wrmsrl/wrmsrq (Xin)
> - return true in is_xstate_managed_msr() for MSR_IA32_S_CET
> ---
>   arch/x86/kvm/x86.c | 36 +++++++++++++++++++++++++++++++++++-
>   arch/x86/kvm/x86.h | 24 ++++++++++++++++++++++++
>   2 files changed, 59 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c15e8c00dc7d..7c0a07be6b64 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -136,6 +136,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>   static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>   
>   static DEFINE_MUTEX(vendor_module_lock);
> +static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> +static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
> +
>   struct kvm_x86_ops kvm_x86_ops __read_mostly;
>   
>   #define KVM_X86_OP(func)					     \
> @@ -4566,6 +4569,22 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   }
>   EXPORT_SYMBOL_GPL(kvm_get_msr_common);
>   
> +/*
> + *  Returns true if the MSR in question is managed via XSTATE, i.e. is context
> + *  switched with the rest of guest FPU state.
> + */
> +static bool is_xstate_managed_msr(u32 index)
> +{
> +	switch (index) {
> +	case MSR_IA32_S_CET:
> +	case MSR_IA32_U_CET:
> +	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> +		return true;
> +	default:
> +		return false;
> +	}
> +}
> +
>   /*
>    * Read or write a bunch of msrs. All parameters are kernel addresses.
>    *
> @@ -4576,11 +4595,26 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
>   		    int (*do_msr)(struct kvm_vcpu *vcpu,
>   				  unsigned index, u64 *data))
>   {
> +	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?

> +		    is_xstate_managed_msr(entries[i].index)) {
> +			kvm_load_guest_fpu(vcpu);
> +			fpu_loaded = true;
> +		}
>   		if (do_msr(vcpu, entries[i].index, &entries[i].data))
>   			break;
> +	}
> +	if (fpu_loaded)
> +		kvm_put_guest_fpu(vcpu);
>   
>   	return i;
>   }
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index eb3088684e8a..34afe43579bb 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -701,4 +701,28 @@ int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, int cpl,
>   
>   int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
>   
> +/*
> + * Lock and/or reload guest FPU and access xstate MSRs. For accesses initiated
> + * by host, guest FPU is loaded in __msr_io(). For accesses initiated by guest,
> + * guest FPU should have been loaded already.
> + */
> +
> +static inline void kvm_get_xstate_msr(struct kvm_vcpu *vcpu,
> +				      struct msr_data *msr_info)
> +{
> +	KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm);
> +	kvm_fpu_get();
> +	rdmsrq(msr_info->index, msr_info->data);
> +	kvm_fpu_put();
> +}
> +
> +static inline void kvm_set_xstate_msr(struct kvm_vcpu *vcpu,
> +				      struct msr_data *msr_info)
> +{
> +	KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm);
> +	kvm_fpu_get();
> +	wrmsrq(msr_info->index, msr_info->data);
> +	kvm_fpu_put();
> +}
> +
>   #endif