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

Xiaoyao Li
2019-03-14T06:38:58Z

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