2020-07-16 03:19:11

by Yang, Weijiang

Subject: [RESEND v13 06/11] KVM: x86: Load guest fpu state when access MSRs managed by XSAVES

From: Sean Christopherson <[email protected]>

A handful of CET MSRs are not context switched through "traditional"
methods, e.g. VMCS or manual switching, but rather are passed through
to the guest and are saved and restored by XSAVES/XRSTORS, i.e. in the
guest's FPU state.

Load the guest's FPU state if userspace is accessing MSRs whose values
are managed by XSAVES so that the MSR helper, e.g. vmx_{get,set}_msr(),
can simply do {RD,WR}MSR to access the guest's value.

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 <[email protected]>
Co-developed-by: Yang Weijiang <[email protected]>
Signed-off-by: Yang Weijiang <[email protected]>
---
arch/x86/kvm/x86.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8aed32ff9c0c..c437ddc22ad6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -107,6 +107,8 @@ static void enter_smm(struct kvm_vcpu *vcpu);
static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
static void store_regs(struct kvm_vcpu *vcpu);
static int sync_regs(struct kvm_vcpu *vcpu);
+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;
EXPORT_SYMBOL_GPL(kvm_x86_ops);
@@ -3356,6 +3358,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
}
EXPORT_SYMBOL_GPL(kvm_get_msr_common);

+static bool is_xsaves_msr(u32 index)
+{
+ return index == MSR_IA32_U_CET ||
+ (index >= MSR_IA32_PL0_SSP && index <= MSR_IA32_PL3_SSP);
+}
+
/*
* Read or write a bunch of msrs. All parameters are kernel addresses.
*
@@ -3366,11 +3374,20 @@ 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 && 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;
}
--
2.17.2

2020-07-22 20:34:01

by Sean Christopherson

Subject: Re: [RESEND v13 06/11] KVM: x86: Load guest fpu state when access MSRs managed by XSAVES

On Thu, Jul 16, 2020 at 11:16:22AM +0800, Yang Weijiang wrote:
> From: Sean Christopherson <[email protected]>
>
> A handful of CET MSRs are not context switched through "traditional"
> methods, e.g. VMCS or manual switching, but rather are passed through
> to the guest and are saved and restored by XSAVES/XRSTORS, i.e. in the
> guest's FPU state.
>
> Load the guest's FPU state if userspace is accessing MSRs whose values
> are managed by XSAVES so that the MSR helper, e.g. vmx_{get,set}_msr(),
> can simply do {RD,WR}MSR to access the guest's value.
>
> 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.

No comments on the patch itself. Added a blurb to the changelog to call
out the vcpu==NULL case is possible due to KVM_GET_MSRS also being a device
scope ioctl().

> Signed-off-by: Sean Christopherson <[email protected]>
> Co-developed-by: Yang Weijiang <[email protected]>
> Signed-off-by: Yang Weijiang <[email protected]>