linux-kernel - Re: [PATCH 08/61] KVM: x86: Warn on zero-size save state for valid CPUID 0xD.N sub-leaf

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Message-ID: <87blqaqtnw.fsf@vitty.brq.redhat.com>
Date:   Fri, 07 Feb 2020 16:54:59 +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 08/61] KVM: x86: Warn on zero-size save state for valid CPUID 0xD.N sub-leaf

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

> WARN if the save state size for a valid XCR0-managed sub-leaf is zero,
> which would indicate a KVM or CPU bug.  Add a comment to explain why KVM
> WARNs so the reader doesn't have to tease out the relevant bits from
> Intel's SDM and KVM's XCR0/XSS code.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index fd9b29aa7abc..424dde41cb5d 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -677,10 +677,17 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  				goto out;
>  
>  			do_host_cpuid(&entry[i], function, idx);
> -			if (entry[i].eax == 0)
> -				continue;
> -			if (WARN_ON_ONCE(entry[i].ecx & 1))
> +
> +			/*
> +			 * The @supported check above should have filtered out
> +			 * invalid sub-leafs as well as sub-leafs managed by

Is it 'sub-leafs' or 'sub-leaves' actually? :-)

> +			 * IA32_XSS MSR.  Only XCR0-managed sub-leafs should
> +			 * reach this point, and they should have a non-zero
> +			 * save state size.
> +			 */
> +			if (WARN_ON_ONCE(!entry[i].eax || (entry[i].ecx & 1)))
>  				continue;
> +
>  			entry[i].ecx = 0;
>  			entry[i].edx = 0;
>  			++*nent;

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

-- 
Vitaly