linux-kernel - Re: [PATCH][next] KVM: x86: simplify zero'ing of entry->ebx

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Date:   Thu, 22 Apr 2021 15:07:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] KVM: x86: simplify zero'ing of entry->ebx

On Thu, Apr 22, 2021, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> Currently entry->ebx is being zero'd by masking itself with zero.
> Simplify this by just assigning zero, cleans up static analysis
> warning.
> 
> Addresses-Coverity: ("Bitwise-and with zero")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  arch/x86/kvm/cpuid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 57744a5d1bc2..9bcc2ff4b232 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -851,7 +851,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		entry->eax &= SGX_ATTR_DEBUG | SGX_ATTR_MODE64BIT |
>  			      SGX_ATTR_PROVISIONKEY | SGX_ATTR_EINITTOKENKEY |
>  			      SGX_ATTR_KSS;
> -		entry->ebx &= 0;
> +		entry->ebx = 0;

I 100% understand the code is funky, but using &= is intentional.  ebx:eax holds
a 64-bit value that is a effectively a set of feature flags.  While the upper
32 bits are extremely unlikely to be used any time soon, if a feature comes
along then the correct behavior would be:

		entry->ebx &= SGX_ATTR_FANCY_NEW_FEATURE;

While directly setting entry->ebx would be incorrect.  The idea is to set up a
future developer for success so that they don't forget to add the "&".

TL;DR: I'd prefer to keep this as is, even though it's rather ridiculous.

>  		break;
>  	/* Intel PT */
>  	case 0x14:
> -- 
> 2.30.2
>