Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting

From: Andy Lutomirski
Date: Fri Jul 27 2018 - 15:41:32 EST

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In reply to: Jim Mattson: "Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting"
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On Wed, Feb 8, 2017 at 12:09 AM, 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>
> Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 2 ++
> arch/x86/kvm/cpuid.c | 3 +++
> arch/x86/kvm/cpuid.h | 11 +++++++++++
> arch/x86/kvm/emulate.c | 7 +++++++
> arch/x86/kvm/x86.c | 26 ++++++++++++++++++++++++++
> 5 files changed, 49 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index a7066dc1a7e9..a0d6f0c47440 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -601,16 +601,18 @@ 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;
> + u64 msr_platform_info;
> + u64 msr_misc_features_enables;
>
> 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 e85f6bd7b9d5..588ac8ae0a60 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -877,16 +877,19 @@ 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);
>
> int kvm_emulate_cpuid(struct kvm_vcpu *vcpu)
> {
> u32 eax, ebx, ecx, edx;
>
> + if (cpuid_fault_enabled(vcpu) && !kvm_require_cpl(vcpu, 0))
> + return;
> +
> 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);
> return kvm_skip_emulated_instruction(vcpu);
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 35058c2c0eea..a6fd40aade7c 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -200,9 +200,20 @@ static inline int guest_cpuid_stepping(struct kvm_vcpu *vcpu)
>
> best = kvm_find_cpuid_entry(vcpu, 0x1, 0);
> if (!best)
> return -1;
>
> return x86_stepping(best->eax);
> }
>
> +static inline bool supports_cpuid_fault(struct kvm_vcpu *vcpu)
> +{
> + return vcpu->arch.msr_platform_info & MSR_PLATFORM_INFO_CPUID_FAULT;
> +}
> +
> +static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)
> +{
> + return vcpu->arch.msr_misc_features_enables &
> + MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
> +}
> +
> #endif
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index cedbba0f3402..8b4b5566c365 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3848,16 +3848,23 @@ static int em_sti(struct x86_emulate_ctxt *ctxt)
> ctxt->interruptibility = KVM_X86_SHADOW_INT_STI;
> ctxt->eflags |= X86_EFLAGS_IF;
> return X86EMUL_CONTINUE;
> }
>
> static int em_cpuid(struct x86_emulate_ctxt *ctxt)
> {
> u32 eax, ebx, ecx, edx;
> + u64 msr = 0;
> +
> + ctxt->ops->get_msr(ctxt, MSR_MISC_FEATURES_ENABLES, &msr);
> + if (msr & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
> + ctxt->ops->cpl(ctxt)) {
> + return emulate_gp(ctxt, 0);
> + }

Not strictly a comment on your patch, but why on Earth do there need
to be two copies of this check?

>
> eax = reg_read(ctxt, VCPU_REGS_RAX);
> ecx = reg_read(ctxt, VCPU_REGS_RCX);
> ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx);
> *reg_write(ctxt, VCPU_REGS_RAX) = eax;
> *reg_write(ctxt, VCPU_REGS_RBX) = ebx;
> *reg_write(ctxt, VCPU_REGS_RCX) = ecx;
> *reg_write(ctxt, VCPU_REGS_RDX) = edx;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e52c9088660f..1951b460da47 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -998,16 +998,18 @@ static u32 emulated_msrs[] = {
>
> MSR_IA32_TSC_ADJUST,
> MSR_IA32_TSCDEADLINE,
> MSR_IA32_MISC_ENABLE,
> MSR_IA32_MCG_STATUS,
> MSR_IA32_MCG_CTL,
> MSR_IA32_MCG_EXT_CTL,
> MSR_IA32_SMBASE,
> + MSR_PLATFORM_INFO,
> + MSR_MISC_FEATURES_ENABLES,
> };
>
> static unsigned num_emulated_msrs;
>
> bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
> {
> if (efer & efer_reserved_bits)
> return false;
> @@ -2285,16 +2287,31 @@ 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_PLATFORM_INFO:
> + if (!msr_info->host_initiated ||
> + data & ~MSR_PLATFORM_INFO_CPUID_FAULT ||
> + (!(data & MSR_PLATFORM_INFO_CPUID_FAULT) &&
> + cpuid_fault_enabled(vcpu)))
> + return 1;
> + vcpu->arch.msr_platform_info = data;
> + break;
> + case MSR_MISC_FEATURES_ENABLES:
> + if (data & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT ||
> + (data & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
> + !supports_cpuid_fault(vcpu)))
> + return 1;
> + vcpu->arch.msr_misc_features_enables = data;
> + 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_debug_ratelimited(vcpu, "unhandled wrmsr: 0x%x data 0x%llx\n",
> msr, data);
> @@ -2499,16 +2516,22 @@ 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:
> + msr_info->data = vcpu->arch.msr_platform_info;
> + break;
> + case MSR_MISC_FEATURES_ENABLES:
> + msr_info->data = vcpu->arch.msr_misc_features_enables;
> + break;
> 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_debug_ratelimited(vcpu, "unhandled rdmsr: 0x%x\n",
> msr_info->index);
> return 1;
> } else {
> @@ -7613,16 +7636,19 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>
> kvm_clear_async_pf_completion_queue(vcpu);
> kvm_async_pf_hash_reset(vcpu);
> vcpu->arch.apf.halted = false;
>
> if (!init_event) {
> kvm_pmu_reset(vcpu);
> vcpu->arch.smbase = 0x30000;
> +
> + vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
> + vcpu->arch.msr_misc_features_enables = 0;

Jim, I assume you're worried about this bit? It seems like
msr_platform_info should maybe be initialized to zero to avoid causing
an unintended migration issue.

Next message: Andy Lutomirski: "Re: [PATCH 29/38] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #10]"
Previous message: Matthew Wilcox: "Re: [PATCH 18/26] target/iscsi: Allocate session IDs from an IDA"
In reply to: Jim Mattson: "Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting"
Next in thread: Jim Mattson: "Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting"
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