Re: [PATCH v8 7/7] KVM: x86: virtualize cpuid faulting

From: David Matlack
Date: Fri Nov 04 2016 - 16:35:12 EST

On Mon, Oct 31, 2016 at 6:37 PM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
> Hardware support for faulting on the cpuid instruction is not required to
> emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant
> MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a
> cpuid-induced VM exit checks the cpuid faulting state and the CPL.
> kvm_require_cpl is even kind enough to inject the GP fault for us.
>
> Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/cpuid.c | 3 +++
> arch/x86/kvm/x86.c | 16 ++++++++++++++++
> 3 files changed, 20 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 4b20f73..4a6e62b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -592,16 +592,17 @@ struct kvm_vcpu_arch {
> u64 pat;
>
> unsigned switch_db_regs;
> unsigned long db[KVM_NR_DB_REGS];
> unsigned long dr6;
> unsigned long dr7;
> unsigned long eff_db[KVM_NR_DB_REGS];
> unsigned long guest_debug_dr7;
> + bool cpuid_fault;
>
> u64 mcg_cap;
> u64 mcg_status;
> u64 mcg_ctl;
> u64 mcg_ext_ctl;
> u64 *mce_banks;
>
> /* Cache MMIO info */
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index afa7bbb..ed8436a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -862,16 +862,19 @@ void kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx, u32 *ecx, u32 *edx)
> trace_kvm_cpuid(function, *eax, *ebx, *ecx, *edx);
> }
> EXPORT_SYMBOL_GPL(kvm_cpuid);
>
> void kvm_emulate_cpuid(struct kvm_vcpu *vcpu)
> {
> u32 function, eax, ebx, ecx, edx;
>
> + if (vcpu->arch.cpuid_fault && !kvm_require_cpl(vcpu, 0))
> + return;
> +
> function = eax = kvm_register_read(vcpu, VCPU_REGS_RAX);
> ecx = kvm_register_read(vcpu, VCPU_REGS_RCX);
> kvm_cpuid(vcpu, &eax, &ebx, &ecx, &edx);
> kvm_register_write(vcpu, VCPU_REGS_RAX, eax);
> kvm_register_write(vcpu, VCPU_REGS_RBX, ebx);
> kvm_register_write(vcpu, VCPU_REGS_RCX, ecx);
> kvm_register_write(vcpu, VCPU_REGS_RDX, edx);
> kvm_x86_ops->skip_emulated_instruction(vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e375235..470c553 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2269,16 +2269,21 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> return 1;
> vcpu->arch.osvw.length = data;
> break;
> case MSR_AMD64_OSVW_STATUS:
> if (!guest_cpuid_has_osvw(vcpu))
> return 1;
> vcpu->arch.osvw.status = data;
> break;
> + case MSR_MISC_FEATURES_ENABLES:
> + if (data & ~CPUID_FAULT_ENABLE)
> + return 1;

(Due to my comments below, PLATINFO_CPUID_FAULT will not necessarily
be enabled for guests. So this code will need to check if the virtual CPU
supports PLATINFO_CPUID_FAULT before enabling CPUID faulting.)

> + vcpu->arch.cpuid_fault = !!(data & CPUID_FAULT_ENABLE);
> + break;
> default:
> if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
> return xen_hvm_config(vcpu, data);
> if (kvm_pmu_is_valid_msr(vcpu, msr))
> return kvm_pmu_set_msr(vcpu, msr_info);
> if (!ignore_msrs) {
> vcpu_unimpl(vcpu, "unhandled wrmsr: 0x%x data %llx\n",
> msr, data);
> @@ -2483,16 +2488,25 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> return 1;
> msr_info->data = vcpu->arch.osvw.length;
> break;
> case MSR_AMD64_OSVW_STATUS:
> if (!guest_cpuid_has_osvw(vcpu))
> return 1;
> msr_info->data = vcpu->arch.osvw.status;
> break;
> + case MSR_PLATFORM_INFO:
> + /* cpuid faulting is supported */
> + msr_info->data = PLATINFO_CPUID_FAULT;
> + break;

This could break save/restore, if for example, a VM is migrated to a
version of KVM without MSR_PLATFORM_INFO support. I think the way to
handle this is to make MSR_PLATFORM_INFO writeable (but only from
userspace) so that hypervisors can defend themselves (by setting this
MSR to 0).

> + case MSR_MISC_FEATURES_ENABLES:
> + msr_info->data = 0;
> + if (vcpu->arch.cpuid_fault)
> + msr_info->data |= CPUID_FAULT_ENABLE;
> + break;

MSR_MISC_FEATURES_ENABLES should be added to emulated_msrs[] so that
the hypervisor will maintain the value of CPUID_FAULT_ENABLE across a
save/restore.

> default:
> if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
> return kvm_pmu_get_msr(vcpu, msr_info->index, &msr_info->data);
> if (!ignore_msrs) {
> vcpu_unimpl(vcpu, "unhandled rdmsr: 0x%x\n", msr_info->index);
> return 1;
> } else {
> vcpu_unimpl(vcpu, "ignored rdmsr: 0x%x\n", msr_info->index);
> @@ -7493,16 +7507,18 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> kvm_update_dr0123(vcpu);
> vcpu->arch.dr6 = DR6_INIT;
> kvm_update_dr6(vcpu);
> vcpu->arch.dr7 = DR7_FIXED_1;
> kvm_update_dr7(vcpu);
>
> vcpu->arch.cr2 = 0;
>
> + vcpu->arch.cpuid_fault = false;
> +
> kvm_make_request(KVM_REQ_EVENT, vcpu);
> vcpu->arch.apf.msr_val = 0;
> vcpu->arch.st.msr_val = 0;
>
> kvmclock_reset(vcpu);
>
> kvm_clear_async_pf_completion_queue(vcpu);
> kvm_async_pf_hash_reset(vcpu);
> --
> 2.10.2
>
> --
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