linux-kernel - Re: [PATCH RFC 13/43] x86/paravirt: Use relative reference for original instruction

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Message-ID: <4D9D12CE-87A7-485F-B8F6-A9F77156C372@vmware.com>
Date:   Mon, 5 Jun 2023 06:40:54 +0000
From:   Nadav Amit <namit@...are.com>
To:     Juergen Gross <jgross@...e.com>,
        Hou Wenlong <houwenlong.hwl@...group.com>
CC:     kernel list <linux-kernel@...r.kernel.org>,
        Thomas Garnier <thgarnie@...omium.org>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Kees Cook <keescook@...omium.org>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        Pv-drivers <Pv-drivers@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Song Liu <song@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH RFC 13/43] x86/paravirt: Use relative reference for
 original instruction



> On Jun 1, 2023, at 2:29 AM, Juergen Gross <jgross@...e.com> wrote:
> 
> On 28.04.23 11:50, Hou Wenlong wrote:
>> Similar to the alternative patching, use relative reference for original
>> instruction rather than absolute one, which saves 8 bytes for one entry
>> on x86_64.  And it could generate R_X86_64_PC32 relocation instead of
>> R_X86_64_64 relocation, which also reduces relocation metadata on
>> relocatable builds. And the alignment could be hard coded to be 4 now.
>> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
>> Cc: Thomas Garnier <thgarnie@...omium.org>
>> Cc: Lai Jiangshan <jiangshan.ljs@...group.com>
>> Cc: Kees Cook <keescook@...omium.org>
> 
> Reviewed-by: Juergen Gross <jgross@...e.com>
> 
> I think this patch should be taken even without the series.

It looks good to me, I am just not sure what the alignment is needed
at all.

Why not to make the struct __packed (like struct alt_instr) and get rid
of all the .align directives? Am I missing something?