[PATCH] kvm: x86: replace kvm_spec_ctrl_test_value with runtime test on the host

Maxim Levitsky
2020-07-08T11:57:45Z

To avoid complex and in some cases incorrect logic in
kvm_spec_ctrl_test_value, just try the guest's given value on the host
processor instead, and if it doesn't #GP, allow the guest to set it.

One such case is when host CPU supports STIBP mitigation
but doesn't support IBRS (as is the case with some Zen2 AMD cpus),
and in this case we were giving guest #GP when it tried to use STIBP

The reason why can can do the host test is that IA32_SPEC_CTRL msr is
passed to the guest, after the guest sets it to a non zero value
for the first time (due to performance reasons),
and as as result of this, it is pointless to emulate #GP condition on
this first access, in a different way than what the host CPU does.

This is based on a patch from Sean Christopherson, who suggested this idea.

Fixes: 6441fa6178f5 ("KVM: x86: avoid incorrect writes to host MSR_IA32_SPEC_CTRL")

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/svm/svm.c | 2 +-
arch/x86/kvm/vmx/vmx.c | 2 +-
arch/x86/kvm/x86.c | 38 +++++++++++++++++++++-----------------
arch/x86/kvm/x86.h | 2 +-
4 files changed, 24 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 74096aa72ad9..80421a72beb0 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2522,7 +2522,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
!guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
return 1;

- if (data & ~kvm_spec_ctrl_valid_bits(vcpu))
+ if (kvm_spec_ctrl_test_value(data))
return 1;

svm->spec_ctrl = data;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 8187ca152ad2..01643893cf8e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2065,7 +2065,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
return 1;

- if (data & ~kvm_spec_ctrl_valid_bits(vcpu))
+ if (kvm_spec_ctrl_test_value(data))
return 1;

vmx->spec_ctrl = data;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 09ee54f5e385..84da4d0cc05a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10706,28 +10706,32 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_arch_no_poll);

-u64 kvm_spec_ctrl_valid_bits(struct kvm_vcpu *vcpu)
+
+int kvm_spec_ctrl_test_value(u64 value)
{
- uint64_t bits = SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD;
+ /*
+ * test that setting IA32_SPEC_CTRL to given value
+ * is allowed by the host processor
+ */
+
+ u64 saved_value;
+ unsigned long flags;
+ int ret = 0;

- /* The STIBP bit doesn't fault even if it's not advertised */
- if (!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
- !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS))
- bits &= ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP);
- if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL) &&
- !boot_cpu_has(X86_FEATURE_AMD_IBRS))
- bits &= ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP);
+ local_irq_save(flags);

- if (!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL_SSBD) &&
- !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
- bits &= ~SPEC_CTRL_SSBD;
- if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) &&
- !boot_cpu_has(X86_FEATURE_AMD_SSBD))
- bits &= ~SPEC_CTRL_SSBD;
+ if (rdmsrl_safe(MSR_IA32_SPEC_CTRL, &saved_value))
+ ret = 1;
+ else if (wrmsrl_safe(MSR_IA32_SPEC_CTRL, value))
+ ret = 1;
+ else
+ wrmsrl(MSR_IA32_SPEC_CTRL, saved_value);

- return bits;
+ local_irq_restore(flags);
+
+ return ret;
}
-EXPORT_SYMBOL_GPL(kvm_spec_ctrl_valid_bits);
+EXPORT_SYMBOL_GPL(kvm_spec_ctrl_test_value);

EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_fast_mmio);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 31928bf18ba5..73780a832691 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -368,7 +368,7 @@ static inline bool kvm_dr6_valid(u64 data)

void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu);
void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu);
-u64 kvm_spec_ctrl_valid_bits(struct kvm_vcpu *vcpu);
+int kvm_spec_ctrl_test_value(u64 value);
bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu);

#define KVM_MSR_RET_INVALID 2
--
2.25.4