Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

[KarimAllah Ahmed <karahmed@amazon.com>]

On 01/28/2018 09:21 PM, Konrad Rzeszutek Wilk wrote:
> On January 28, 2018 2:29:10 PM EST, KarimAllah Ahmed <karahmed@amazon.de> wrote:
>> Add direct access to MSR_IA32_SPEC_CTRL for guests. This is needed for
>> guests
>> that will only mitigate Spectre V2 through IBRS+IBPB and will not be
>> using a
>> retpoline+IBPB based approach.
>>
>> To avoid the overhead of atomically saving and restoring the
>> MSR_IA32_SPEC_CTRL
>> for guests that do not actually use the MSR, only add_atomic_switch_msr
>> when a
>> non-zero is written to it.
> 
> 
> We tried this and found that it was about 3% slower that doing the old way of rdmsr and wrmsr.

I actually have not measured the performance difference between using 
the atomic_switch vs just just doing rdmsr/wrmsr. I was mostly focused 
on not saving and restoring when the guest does not actually use the MSRs.

Interesting data point though, I will update the code to use rdmsr/wrmsr 
and see if I see it in my hardware (I am using a skylake processor).

> 
> But that was also with the host doing IBRS  as well.
> 
> On what type of hardware did you run this?
> 
> Ccing Daniel.
>>
>> Cc: Asit Mallick <asit.k.mallick@intel.com>
>> Cc: Arjan Van De Ven <arjan.van.de.ven@intel.com>
>> Cc: Dave Hansen <dave.hansen@intel.com>
>> Cc: Andi Kleen <ak@linux.intel.com>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Tim Chen <tim.c.chen@linux.intel.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Jun Nakajima <jun.nakajima@intel.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: David Woodhouse <dwmw@amazon.co.uk>
>> Cc: Greg KH <gregkh@linuxfoundation.org>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
>> Signed-off-by: Ashok Raj <ashok.raj@intel.com>
>> ---
>> arch/x86/kvm/cpuid.c |  4 +++-
>> arch/x86/kvm/cpuid.h |  1 +
>> arch/x86/kvm/vmx.c   | 63
>> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 67 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 0099e10..dc78095 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -70,6 +70,7 @@ u64 kvm_supported_xcr0(void)
>> /* These are scattered features in cpufeatures.h. */
>> #define KVM_CPUID_BIT_AVX512_4VNNIW     2
>> #define KVM_CPUID_BIT_AVX512_4FMAPS     3
>> +#define KVM_CPUID_BIT_SPEC_CTRL         26
>> #define KF(x) bit(KVM_CPUID_BIT_##x)
>>
>> int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>> @@ -392,7 +393,8 @@ static inline int __do_cpuid_ent(struct
>> kvm_cpuid_entry2 *entry, u32 function,
>>
>> 	/* cpuid 7.0.edx*/
>> 	const u32 kvm_cpuid_7_0_edx_x86_features =
>> -		KF(AVX512_4VNNIW) | KF(AVX512_4FMAPS);
>> +		KF(AVX512_4VNNIW) | KF(AVX512_4FMAPS) | \
>> +		(boot_cpu_has(X86_FEATURE_SPEC_CTRL) ? KF(SPEC_CTRL) : 0);
>>
>> 	/* all calls to cpuid_count() should be made on the same cpu */
>> 	get_cpu();
>> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
>> index cdc70a3..dcfe227 100644
>> --- a/arch/x86/kvm/cpuid.h
>> +++ b/arch/x86/kvm/cpuid.h
>> @@ -54,6 +54,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
>> 	[CPUID_8000_000A_EDX] = {0x8000000a, 0, CPUID_EDX},
>> 	[CPUID_7_ECX]         = {         7, 0, CPUID_ECX},
>> 	[CPUID_8000_0007_EBX] = {0x80000007, 0, CPUID_EBX},
>> +	[CPUID_7_EDX]         = {         7, 0, CPUID_EDX},
>> };
>>
>> static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned
>> x86_feature)
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index aa8638a..1b743a0 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -920,6 +920,9 @@ static void vmx_set_nmi_mask(struct kvm_vcpu *vcpu,
>> bool masked);
>> static bool nested_vmx_is_page_fault_vmexit(struct vmcs12 *vmcs12,
>> 					    u16 error_code);
>> static void vmx_update_msr_bitmap(struct kvm_vcpu *vcpu);
>> +static void __always_inline vmx_disable_intercept_for_msr(unsigned
>> long *msr_bitmap,
>> +							  u32 msr, int type);
>> +
>>
>> static DEFINE_PER_CPU(struct vmcs *, vmxarea);
>> static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
>> @@ -2007,6 +2010,28 @@ static void add_atomic_switch_msr(struct
>> vcpu_vmx *vmx, unsigned msr,
>> 	m->host[i].value = host_val;
>> }
>>
>> +/* do not touch guest_val and host_val if the msr is not found */
>> +static int read_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
>> +				  u64 *guest_val, u64 *host_val)
>> +{
>> +	unsigned i;
>> +	struct msr_autoload *m = &vmx->msr_autoload;
>> +
>> +	for (i = 0; i < m->nr; ++i)
>> +		if (m->guest[i].index == msr)
>> +			break;
>> +
>> +	if (i == m->nr)
>> +		return 1;
>> +
>> +	if (guest_val)
>> +		*guest_val = m->guest[i].value;
>> +	if (host_val)
>> +		*host_val = m->host[i].value;
>> +
>> +	return 0;
>> +}
>> +
>> static bool update_transition_efer(struct vcpu_vmx *vmx, int
>> efer_offset)
>> {
>> 	u64 guest_efer = vmx->vcpu.arch.efer;
>> @@ -3203,7 +3228,9 @@ static inline bool
>> vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
>>   */
>> static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data
>> *msr_info)
>> {
>> +	u64 spec_ctrl = 0;
>> 	struct shared_msr_entry *msr;
>> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>
>> 	switch (msr_info->index) {
>> #ifdef CONFIG_X86_64
>> @@ -3223,6 +3250,19 @@ 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:
>> +		if (!msr_info->host_initiated &&
>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
>> +			return 1;
>> +
>> +		/*
>> +		 * If the MSR is not in the atomic list yet, then it was never
>> +		 * written to. So the MSR value will be '0'.
>> +		 */
>> +		read_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, &spec_ctrl, NULL);
>> +
>> +		msr_info->data = spec_ctrl;
>> +		break;
>> 	case MSR_IA32_SYSENTER_CS:
>> 		msr_info->data = vmcs_read32(GUEST_SYSENTER_CS);
>> 		break;
>> @@ -3289,6 +3329,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu,
>> struct msr_data *msr_info)
>> 	int ret = 0;
>> 	u32 msr_index = msr_info->index;
>> 	u64 data = msr_info->data;
>> +	unsigned long *msr_bitmap;
>> +
>> +	/*
>> +	 * IBRS is not used (yet) to protect the host. Once it does, this
>> +	 * variable needs to be a bit smarter.
>> +	 */
>> +	u64 host_spec_ctrl = 0;
>>
>> 	switch (msr_index) {
>> 	case MSR_EFER:
>> @@ -3330,6 +3377,22 @@ 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:
>> +		if (!msr_info->host_initiated &&
>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
>> +			return 1;
>> +
>> +		/*
>> +		 * Now we know that the guest is actually using the MSR, so
>> +		 * atomically load and save the SPEC_CTRL MSR and pass it
>> +		 * through to the guest.
>> +		 */
>> +		add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, msr_info->data,
>> +				      host_spec_ctrl);
>> +		msr_bitmap = vmx->vmcs01.msr_bitmap;
>> +		vmx_disable_intercept_for_msr(msr_bitmap, MSR_FS_BASE, MSR_TYPE_RW);
>> +
>> +		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))
> 
> 
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