[v7,15/26] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs
Commit Message
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>
---
arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++++++++-
arch/x86/kvm/x86.h | 24 ++++++++++++++++++++++++
2 files changed, 58 insertions(+), 1 deletion(-)
Comments
On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang 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>
> ---
> arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++++++++-
> arch/x86/kvm/x86.h | 24 ++++++++++++++++++++++++
> 2 files changed, 58 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 607082aca80d..fd48b825510c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -133,6 +133,9 @@ 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) \
> @@ -4482,6 +4485,21 @@ 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_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.
> *
> @@ -4492,11 +4510,26 @@ 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 &&
> + 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 5184fde1dc54..6e42ede335f5 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -541,4 +541,28 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
> unsigned int port, void *data, unsigned int count,
> int in);
>
> +/*
> + * 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();
> + rdmsrl(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();
> + wrmsrl(msr_info->index, msr_info->data);
> + kvm_fpu_put();
> +}
> +
> #endif
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Best regards,
Maxim Levitsky
@@ -133,6 +133,9 @@ 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) \
@@ -4482,6 +4485,21 @@ 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_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.
*
@@ -4492,11 +4510,26 @@ 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 &&
+ 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;
}
@@ -541,4 +541,28 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
unsigned int port, void *data, unsigned int count,
int in);
+/*
+ * 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();
+ rdmsrl(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();
+ wrmsrl(msr_info->index, msr_info->data);
+ kvm_fpu_put();
+}
+
#endif