linux-kernel - Re: [PATCH v4 03/21] kvm: x86: Fix xstate_required

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Message-ID: <YdSl4s78kj/U/5Bl@google.com>
Date:   Tue, 4 Jan 2022 19:54:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Zhong <yang.zhong@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, pbonzini@...hat.com, corbet@....net,
        shuah@...nel.org, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com,
        guang.zeng@...el.com, wei.w.wang@...el.com
Subject: Re: [PATCH v4 03/21] kvm: x86: Fix xstate_required_size() to follow
 XSTATE alignment rule

On Wed, Dec 29, 2021, Yang Zhong wrote:
> From: Jing Liu <jing2.liu@...el.com>
> 
> CPUID.0xD.1.EBX enumerates the size of the XSAVE area (in compacted
> format) required by XSAVES. If CPUID.0xD.i.ECX[1] is set for a state
> component (i), this state component should be located on the next
> 64-bytes boundary following the preceding state component in the
> compacted layout.
> 
> Fix xstate_required_size() to follow the alignment rule. AMX is the
> first state component with 64-bytes alignment to catch this bug.
> 
> Signed-off-by: Jing Liu <jing2.liu@...el.com>
> Signed-off-by: Yang Zhong <yang.zhong@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 07e9215e911d..148003e26cbb 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -42,7 +42,8 @@ static u32 xstate_required_size(u64 xstate_bv, bool compacted)
>  		if (xstate_bv & 0x1) {
>  		        u32 eax, ebx, ecx, edx, offset;
>  		        cpuid_count(0xD, feature_bit, &eax, &ebx, &ecx, &edx);
> -			offset = compacted ? ret : ebx;
> +			/* ECX[1]: 64B alignment in compacted form */
> +			offset = compacted ? ((ecx & 0x2) ? ALIGN(ret, 64) : ret) : ebx;

That is impressively difficult to read.

			if (compacted)
				offset = (ecx & 0x2) ? ALIGN(ret, 64) : ret;
			else
				offset = ebx;

>  			ret = max(ret, offset + eax);
>  		}
>