[PATCH] kvm/x86/vmx: switch MSR_MISC_FEATURES_ENABLES between host and guest

Xiaoyao Li
2019-03-14T06:50:19Z

CPUID Faulting is a feature about CPUID instruction. When CPUID Faulting is
enabled, all execution of the CPUID instruction outside system-management
mode (SMM) cause a general-protection (#GP) if the CPL > 0.

About this feature, detailed information can be found at
https://www.intel.com/content/dam/www/public/us/en/documents/application-notes/virtualization-technology-flexmigration-application-note.pdf

There is an issue that current kvm doesn't switch the value of
MSR_MISC_FEATURES_ENABLES between host and guest. If MSR_MISC_FEATURES_ENABLES
exists on the hardware cpu, and host enables CPUID faulting (setting the bit 0
of MSR_MISC_FEATURES_ENABLES), it will impact the guest's behavior because
cpuid faulting is enabled by host and passed to guest.

From my tests, when host enables cpuid faulting, it causes guest boot failure
when guest uses *modprobe* to load modules. Below is the error log:

[ 1.233556] traps: modprobe[71] general protection fault ip:7f0077f6495c sp:7ffda148d808 error:0 in ld-2.17.so[7f0077f4d000+22000]
[ 1.237780] traps: modprobe[73] general protection fault ip:7fad5aba095c sp:7ffd36067378 error:0 in ld-2.17.so[7fad5ab89000+22000]
[ 1.241930] traps: modprobe[75] general protection fault ip:7f3edb89495c sp:7fffa1a81308 error:0 in ld-2.17.so[7f3edb87d000+22000]
[ 1.245998] traps: modprobe[77] general protection fault ip:7f91d670895c sp:7ffc25fa7f38 error:0 in ld-2.17.so[7f91d66f1000+22000]
[ 1.250016] traps: modprobe[79] general protection fault ip:7f0ddbbdc95c sp:7ffe9c34f8d8 error:0 in ld-2.17.so[7f0ddbbc5000+22000]

*modprobe* calls CPUID instruction thus causing cpuid faulting in guest.
At the end, because guest cannot *modprobe* modules, it boots failure.

This patch switches MSR_MISC_FEATURES_ENABLES between host and guest when
hardware has this MSR.

This patch doesn't confict with the commit db2336a80489 ("KVM: x86: virtualize
cpuid faulting"), which provides a software emulation of cpuid faulting for
x86 arch. Below analysing how cpuid faulting will work after applying this patch:

1. If host cpu is AMD. It doesn't have MSR_MISC_FEATURES_ENABLES, so we can just
use the software emulation of cpuid faulting.

2. If host cpu is Intel and it doesn't have MSR_MISC_FEATURES_ENABLES. The same
as case 1, we can just use the software emulation of cpuid faulting.

3. If host cpu is Intel and it has MSR_MISC_FEATURES_ENABLES. With this patch,
it will write guest's value into MSR_MISC_FEATURES_ENABLES when vm entry.
If guest enables cpuid faulting and when guest calls CPUID instruction with
CPL > 0, it will cause #GP exception in guest instead of VM exit because of
CPUID, thus it doesn't go to the kvm emualtion path but ues the hardware
feature. Also it's a benefit that we needn't use VM exit to inject #GP to
emulate cpuid faulting feature.

Intel SDM vol3.25.1.1 specifies the priority between cpuid faulting
and CPUID instruction.

Signed-off-by: Xiaoyao Li <xiaoyao.li@linux.intel.com>
---
arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 30a6bcd735ec..90707fae688e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6321,6 +6321,23 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
msrs[i].host, false);
}

+static void atomic_switch_msr_misc_features_enables(struct kvm_vcpu *vcpu)
+{
+ u64 host_msr;
+ struct vcpu_vmx *vmx = to_vmx(vcpu);
+
+ /* if MSR MISC_FEATURES_ENABLES doesn't exist on the hardware, do nothing*/
+ if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &host_msr))
+ return;
+
+ if (host_msr == vcpu->arch.msr_misc_features_enables)
+ clear_atomic_switch_msr(vmx, MSR_MISC_FEATURES_ENABLES);
+ else
+ add_atomic_switch_msr(vmx, MSR_MISC_FEATURES_ENABLES,
+ vcpu->arch.msr_misc_features_enables,
+ host_msr, false);
+}
+
static void vmx_arm_hv_timer(struct vcpu_vmx *vmx, u32 val)
{
vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, val);
@@ -6562,6 +6579,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)

atomic_switch_perf_msrs(vmx);

+ atomic_switch_msr_misc_features_enables(vcpu);
+
vmx_update_hv_timer(vcpu);

/*
--
2.19.1