linux-kernel - Re: VMX HOST_RIP target alignment

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Message-ID: <20221117232208.26ha4tjizeegcbwr@treble>
Date:   Thu, 17 Nov 2022 15:22:08 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, x86@...nel.org
Subject: Re: VMX HOST_RIP target alignment

On Thu, Nov 17, 2022 at 11:46:09PM +0300, Alexey Dobriyan wrote:
> One of the side effects of fixing retbleed for VMX was demoting
> HOST_RIP target from honorable function to a lowly label:
> 
> 	-SYM_FUNC_START(vmx_vmexit)
> 	+SYM_INNER_LABEL(vmx_vmexit, SYM_L_GLOBAL)
> 
> ffffffff81243c49:       0f 01 c2                vmlaunch
> ffffffff81243c4c:       e9 a7 00 00 00          jmp    ffffffff81243cf8 <vmx_vmexit+0xa7>
> 
> ffffffff81243c51 <vmx_vmexit>:
> ffffffff81243c51:       50                      push   rax
> 
> Now I've never measured VM exit latency but is it important to align it
> at 16 bytes like Intel recommends for functions?

Yes, I'd think we should restore the alignment to 16 bytes again, that
change was definitely not intentional.

-- 
Josh