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Message-ID: <be3459db-d972-4d46-a48a-2fab1cde7faa@intel.com>
Date: Wed, 10 Sep 2025 17:37:50 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: acme@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
hpa@...or.com, john.allen@....com, mingo@...nel.org, mingo@...hat.com,
minipli@...ecurity.net, mlevitsk@...hat.com, namhyung@...nel.org,
pbonzini@...hat.com, prsampat@....com, rick.p.edgecombe@...el.com,
seanjc@...gle.com, shuah@...nel.org, tglx@...utronix.de,
weijiang.yang@...el.com, x86@...nel.org, xin@...or.com
Subject: Re: [PATCH v14 06/22] KVM: x86: Load guest FPU state when access
XSAVE-managed MSRs
On 9/9/2025 5:39 PM, Chao Gao wrote:
> From: Sean Christopherson <seanjc@...gle.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@...gle.com>
> Co-developed-by: Yang Weijiang <weijiang.yang@...el.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.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
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