linux-kernel - Re: [PATCH 30/61] KVM: x86: Handle MPX CPUID adjustment in VMX code

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Date:   Mon, 24 Feb 2020 16:14:56 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 30/61] KVM: x86: Handle MPX CPUID adjustment in VMX code

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Move the MPX CPUID adjustments into VMX to eliminate an instance of the
> undesirable "unsigned f_* = *_supported ? F(*) : 0" pattern in the
> common CPUID handling code.
>
> Note, VMX must manually check for kernel support via
> boot_cpu_has(X86_FEATURE_MPX).
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.c   |  3 +--
>  arch/x86/kvm/vmx/vmx.c | 14 ++++++++++++--
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index cb5870a323cc..09e24d1d731c 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -340,7 +340,6 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>  static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry)
>  {
>  	unsigned f_invpcid = kvm_x86_ops->invpcid_supported() ? F(INVPCID) : 0;
> -	unsigned f_mpx = kvm_mpx_supported() ? F(MPX) : 0;
>  	unsigned f_umip = kvm_x86_ops->umip_emulated() ? F(UMIP) : 0;
>  	unsigned f_intel_pt = kvm_x86_ops->pt_supported() ? F(INTEL_PT) : 0;
>  	unsigned f_la57;
> @@ -349,7 +348,7 @@ static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry)
>  	/* cpuid 7.0.ebx */
>  	const u32 kvm_cpuid_7_0_ebx_x86_features =
>  		F(FSGSBASE) | F(BMI1) | F(HLE) | F(AVX2) | F(SMEP) |
> -		F(BMI2) | F(ERMS) | f_invpcid | F(RTM) | f_mpx | F(RDSEED) |
> +		F(BMI2) | F(ERMS) | f_invpcid | F(RTM) | 0 /*MPX*/ | F(RDSEED) |
>  		F(ADX) | F(SMAP) | F(AVX512IFMA) | F(AVX512F) | F(AVX512PF) |
>  		F(AVX512ER) | F(AVX512CD) | F(CLFLUSHOPT) | F(CLWB) | F(AVX512DQ) |
>  		F(SHA_NI) | F(AVX512BW) | F(AVX512VL) | f_intel_pt;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3ff830e2258e..143193fc178e 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7106,8 +7106,18 @@ static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
>  
>  static void vmx_set_supported_cpuid(struct kvm_cpuid_entry2 *entry)
>  {
> -	if (entry->function == 1 && nested)
> -		entry->ecx |= feature_bit(VMX);
> +	switch (entry->function) {
> +	case 0x1:
> +		if (nested)
> +			cpuid_entry_set(entry, X86_FEATURE_VMX);
> +		break;
> +	case 0x7:
> +		if (boot_cpu_has(X86_FEATURE_MPX) && kvm_mpx_supported())
> +			cpuid_entry_set(entry, X86_FEATURE_MPX);
> +		break;
> +	default:
> +		break;
> +	}
>  }
>  
>  static void vmx_request_immediate_exit(struct kvm_vcpu *vcpu)

The word 'must' in the description seems to work like a trigger for
reviewers, their brains automatically turn into 'and what if not?' mode
:-)

So do I understand correctly that kvm_mpx_supported() (which checks for
XFEATURE_MASK_BNDREGS/XFEATURE_MASK_BNDCSR) may actually return true
while 'boot_cpu_has(X86_FEATURE_MPX)' is false? Is this done on purpose,
i.e. why don't we filter these out from vmcs_config early, similar to
SVM?

The patch itself looks good, so
Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly