[PATCH v3 09/21] KVM:x86: Load guest FPU state when accessing xsaves-managed MSRs

Yang Weijiang
2023-05-11T07:14:28Z

From: Sean Christopherson <sean.j.christopherson@intel.com>

Load the guest's FPU state if userspace is accessing MSRs whose values are
managed by XSAVES. Two MSR access helpers, i.e., kvm_{get,set}_xsave_msr(),
are introduced by a later patch to facilitate access to this kind of MSRs.

If new feature MSRs supported in XSS are passed through to the guest they
are saved and restored by {XSAVES|XRSTORS} to/from guest's FPU state at
vm-entry/exit.

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 XSS supporting 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.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Co-developed-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
---
arch/x86/kvm/x86.c | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d2975ca96ac5..7788646bbf1f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -130,6 +130,9 @@ 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) \
@@ -4336,6 +4339,21 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
}
EXPORT_SYMBOL_GPL(kvm_get_msr_common);

+static const u32 xsave_msrs[] = {
+ MSR_IA32_U_CET, MSR_IA32_PL3_SSP,
+};
+
+static bool is_xsaves_msr(u32 index)
+{
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(xsave_msrs); i++) {
+ if (index == xsave_msrs[i])
+ return true;
+ }
+ return false;
+}
+
/*
* Read or write a bunch of msrs. All parameters are kernel addresses.
*
@@ -4346,11 +4364,20 @@ 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 (vcpu && !fpu_loaded && kvm_caps.supported_xss &&
+ is_xsaves_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;
}
--
2.27.0