Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest
From: Liran Alon
Date: Mon Jan 08 2018 - 15:25:01 EST
On 08/01/18 21:18, Jim Mattson wrote:
Guest usage of MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD should be
predicated on guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL).
On Mon, Jan 8, 2018 at 10:08 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
Direct access to MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD is important
for performance. Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
IBRS on VM entry and set it to 0 on VM exit (because Linux does not use
it yet).
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/vmx.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 669f5f74857d..d00bcad7336e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -120,6 +120,8 @@
module_param_named(preemption_timer, enable_preemption_timer, bool, S_IRUGO);
#endif
+static bool __read_mostly have_spec_ctrl;
+
#define KVM_GUEST_CR0_MASK (X86_CR0_NW | X86_CR0_CD)
#define KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST (X86_CR0_WP | X86_CR0_NE)
#define KVM_VM_CR0_ALWAYS_ON \
@@ -609,6 +611,8 @@ struct vcpu_vmx {
u64 msr_host_kernel_gs_base;
u64 msr_guest_kernel_gs_base;
#endif
+ u64 spec_ctrl;
+
u32 vm_entry_controls_shadow;
u32 vm_exit_controls_shadow;
u32 secondary_exec_control;
@@ -3361,6 +3365,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_IA32_TSC:
msr_info->data = guest_read_tsc(vcpu);
break;
+ case MSR_IA32_SPEC_CTRL:
+ msr_info->data = to_vmx(vcpu)->spec_ctrl;
+ break;
I have:
if (!have_spec_ctrl ||
(!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
return 1;
msr_info->data = to_vmx(vcpu)->msr_ia32_spec_ctrl;
break;
case MSR_IA32_SYSENTER_CS:
msr_info->data = vmcs_read32(GUEST_SYSENTER_CS);
break;
@@ -3500,6 +3507,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_IA32_TSC:
kvm_write_tsc(vcpu, msr_info);
break;
+ case MSR_IA32_SPEC_CTRL:
+ to_vmx(vcpu)->spec_ctrl = msr_info->data;
+ break;
I have:
if (!have_spec_ctrl ||
(!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
return 1;
to_vmx(vcpu)->msr_ia32_spec_ctrl = data;
break;
case MSR_IA32_CR_PAT:
if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
@@ -7062,6 +7072,17 @@ static __init int hardware_setup(void)
goto out;
}
+ /*
+ * FIXME: this is only needed until SPEC_CTRL is supported
+ * by upstream Linux in cpufeatures, then it can be replaced
+ * with static_cpu_has.
+ */
+ have_spec_ctrl = cpu_has_spec_ctrl();
+ if (have_spec_ctrl)
+ pr_info("kvm: SPEC_CTRL available\n");
+ else
+ pr_info("kvm: SPEC_CTRL not available\n");
+
if (boot_cpu_has(X86_FEATURE_NX))
kvm_enable_efer_bits(EFER_NX);
@@ -7131,6 +7152,8 @@ static __init int hardware_setup(void)
vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_CS, false);
vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_ESP, false);
vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_EIP, false);
+ vmx_disable_intercept_for_msr(MSR_IA32_SPEC_CTRL, false);
+ vmx_disable_intercept_for_msr(MSR_IA32_PRED_CMD, false);
I have a lot of changes to MSR permission bitmap handling, but these
intercepts should only be disabled when guest_cpuid_has(vcpu,
X86_FEATURE_SPEC_CTRL).
memcpy(vmx_msr_bitmap_legacy_x2apic_apicv,
vmx_msr_bitmap_legacy, PAGE_SIZE);
@@ -9597,6 +9620,9 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
pt_guest_enter(vmx);
+ if (have_spec_ctrl && vmx->spec_ctrl != 0)
+ wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
+
Here, I have:
/*
* If the guest is allowed to write to MSR_IA32_SPEC_CTRL,
* store it on VM-exit.
*/
if (guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
add_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
else
clear_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
/*
* If the guest's IA32_SPEC_CTRL MSR doesn't match the host's
* IA32_SPEC_CTRL MSR, then add the MSR to the atomic switch
* MSRs, so that the guest value will be loaded on VM-entry
* and the host value will be loaded on VM-exit.
*/
if (vmx->msr_ia32_spec_ctrl != spec_ctrl_enabled())
add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL,
vmx->msr_ia32_spec_ctrl,
spec_ctrl_enabled());
else
clear_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL);
I totally agree with this.
This exactly solves the issue I mentioned before of restoring the guest
value of MSR_IA32_SPEC_CTRL using WRMSR before calling
atomic_switch_perf_msrs() which does an indirect branch.
atomic_switch_perf_msrs(vmx);
vmx_arm_hv_timer(vcpu);
@@ -9707,6 +9733,12 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
#endif
);
+ if (have_spec_ctrl) {
+ rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
+ if (vmx->spec_ctrl)
+ wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+ }
+
I know the VM-exit MSR load and store lists are probably slower, but
I'm a little uncomfortable restoring the host's IA32_SPEC_CTRL MSR
late if the guest has it clear and the host has it set.
Again, I totally agree. This is a better approach for handling this.
/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
if (vmx->host_debugctlmsr)
update_debugctlmsr(vmx->host_debugctlmsr);
--
1.8.3.1