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

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Date:   Tue, 06 Jun 2023 19:35:36 +0800
From:   "Hou Wenlong" <houwenlong.hwl@...group.com>
To:     Nadav Amit <namit@...are.com>
Cc:     "Juergen Gross" <jgross@...e.com>,
        "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 Mon, Jun 05, 2023 at 02:40:54PM +0800, Nadav Amit wrote:
> 
> 
> > 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?

Yes, making the struct __packed can save more space. If I understand
correctly, it could be done even without this patch but it may lead to
misaligned memory access. However, it seems to not matter as I didn't
find any related log for packing struct alt_instr. I can do such things
if needed.

Thanks.