linux-kernel - Re: [PATCH 06/61] KVM: x86: Move CPUID 0xD.1 handling out of the index>0 loop

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Date:   Fri, 07 Feb 2020 16:38:02 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 06/61] KVM: x86: Move CPUID 0xD.1 handling out of the index>0 loop

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

> Mov the sub-leaf 1 handling for CPUID 0xD out of the index>0 loop so
> that the loop only handles index>2.  Sub-leafs 2+ have identical
> semantics, whereas sub-leaf 1 is effectively a feature sub-leaf.
>
> Moving sub-leaf 1 out of the loop does duplicate a bit of code, but
> the nent/maxnent code will be consolidated in a future patch, and
> duplicating the clear of ECX/EDX is arguably a good thing as the reasons
> for clearing said registers are completely different.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 37 ++++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index e5cf1e0cf84a..fc8540596386 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -653,26 +653,33 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  		if (!supported)
>  			break;
>  
> -		for (idx = 1, i = 1; idx < 64; ++idx) {
> +		if (*nent >= maxnent)
> +			goto out;
> +
> +		do_host_cpuid(&entry[1], function, 1);
> +		++*nent;
> +
> +		entry[1].eax &= kvm_cpuid_D_1_eax_x86_features;
> +		cpuid_mask(&entry[1].eax, CPUID_D_1_EAX);
> +		if (entry[1].eax & (F(XSAVES)|F(XSAVEC)))
> +			entry[1].ebx = xstate_required_size(supported, true);
> +		else
> +			entry[1].ebx = 0;
> +		/* Saving XSS controlled state via XSAVES isn't supported. */
> +		entry[1].ecx = 0;
> +		entry[1].edx = 0;
> +
> +		for (idx = 2, i = 2; idx < 64; ++idx) {
>  			u64 mask = ((u64)1 << idx);
> +
>  			if (*nent >= maxnent)
>  				goto out;
>  
>  			do_host_cpuid(&entry[i], function, idx);
> -			if (idx == 1) {
> -				entry[i].eax &= kvm_cpuid_D_1_eax_x86_features;
> -				cpuid_mask(&entry[i].eax, CPUID_D_1_EAX);
> -				entry[i].ebx = 0;
> -				if (entry[i].eax & (F(XSAVES)|F(XSAVEC)))
> -					entry[i].ebx =
> -						xstate_required_size(supported,
> -								     true);
> -			} else {
> -				if (entry[i].eax == 0 || !(supported & mask))
> -					continue;
> -				if (WARN_ON_ONCE(entry[i].ecx & 1))
> -					continue;
> -			}
> +			if (entry[i].eax == 0 || !(supported & mask))
> +				continue;
> +			if (WARN_ON_ONCE(entry[i].ecx & 1))
> +				continue;
>  			entry[i].ecx = 0;
>  			entry[i].edx = 0;
>  			++*nent;

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly