Re: [PATCH v2 2/4] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

[Paolo Bonzini]

On 29/01/2018 01:58, KarimAllah Ahmed 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.

You are not storing the guest's MSR value on though vmexit, aren't you?
Also, there's an obvious typo here:

+		add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, msr_info->data, 0);
+
+		msr_bitmap = vmx->vmcs01.msr_bitmap;
+		vmx_disable_intercept_for_msr(msr_bitmap, MSR_FS_BASE, MSR_TYPE_RW);
+

Finally, apparently add_atomic_switch_msr is slower than just rdmsr/wrmsr
on vmexit.  Can you reuse the patches I had posted mid January instead?  They
are also assuming no IBRS usage on the host, so the changes shouldn't be large,
and limited mostly to using actual X86_FEATURE_* bits instead of cpuid_count().

They lack the code to only read/write SPEC_CTRL if the direct access is enabled,
but that's small too...  Enabling the direct access on the first write, as in
this patches, is okay.

Thanks,

Paolo

> Cc: Asit Mallick <asit.k.mallick@xxxxxxxxx>
> Cc: Arjan Van De Ven <arjan.van.de.ven@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> 
> ---
> v2:
> - remove 'host_spec_ctrl' in favor of only a comment (dwmw@).
> - special case writing '0' in SPEC_CTRL to avoid confusing live-migration
>   when the instance never used the MSR (dwmw@).
> - depend on X86_FEATURE_IBRS instead of X86_FEATURE_SPEC_CTRL (dwmw@).
> - add MSR_IA32_SPEC_CTRL to the list of MSRs to save (dropped it by accident).
> ---
>  arch/x86/kvm/cpuid.c |  4 +++-
>  arch/x86/kvm/vmx.c   | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c   |  1 +
>  3 files changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0099e10..32c0c14 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_IBRS              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_IBRS) ? KF(IBRS) : 0);
>  
>  	/* all calls to cpuid_count() should be made on the same cpu */
>  	get_cpu();
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index aa8638a..dac564d 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -920,6 +920,8 @@ 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 +2009,28 @@ 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 +3227,9 @@ 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 +3249,20 @@ 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 the guest
> +		 * never wrote a non-zero value to it yet i.e. the MSR value is
> +		 * '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 +3329,7 @@ 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;
>  
>  	switch (msr_index) {
>  	case MSR_EFER:
> @@ -3330,6 +3371,30 @@ 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_IBRS))
> +			return 1;
> +
> +		if (!msr_info->data)
> +			break;
> +
> +		/*
> +		 * 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.
> +		 *
> +		 * NOTE:
> +		 * IBRS is not supported yet as a mitigation for the host. Once
> +		 * it is supported, the "host_value" will need to be '1'
> +		 * instead of '0' if IBRS is used also by the host.
> +		 */
> +		add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, msr_info->data, 0);
> +
> +		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))
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb..cabaad3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1006,6 +1006,7 @@ static u32 msrs_to_save[] = {
>  #endif
>  	MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
>  	MSR_IA32_FEATURE_CONTROL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
> +	MSR_IA32_SPEC_CTRL
>  };
>  
>  static unsigned num_msrs_to_save;
>