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

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Date:   Thu, 24 Nov 2022 12:18:29 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Hou Wenlong <houwenlong.hwl@...group.com>
Cc:     linux-kernel@...r.kernel.org,
        "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        VMware PV-Drivers Reviewers <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@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>, Song Liu <song@...nel.org>,
        Nadav Amit <namit@...are.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] x86/paravirt: Use relative reference for original
 instruction

On 24.11.22 12:06, Hou Wenlong wrote:
> On Thu, Nov 24, 2022 at 11:18:52AM +0100, Juergen Gross wrote:
>> On 24.11.22 09:51, 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
>>
>> 8 bytes saved? I think those are 4 bytes only.
>>
> The corresponding C structure paravirt_patch_site is not packed, Before this,
> its size is 16 bytes in x86_64，due to the alignment of 8 bytes. Now the alignment
> is 4 bytes, so the size is 8 bytes.

Oh, I've looked at Linus' tree. You seem to have based your reasoning on
the tip/paravirt branch, which is fine.

> 
>>> R_X86_64_64 relocation, which also reduces relocation metadata on
>>> relocatable builds.
>>>
>>> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
>>> ---
>>>   arch/x86/include/asm/paravirt.h       | 6 +++---
>>>   arch/x86/include/asm/paravirt_types.h | 4 ++--
>>>   arch/x86/kernel/alternative.c         | 8 +++++---
>>>   3 files changed, 10 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
>>> index 2851bc2339d5..2cbe9b64e103 100644
>>> --- a/arch/x86/include/asm/paravirt.h
>>> +++ b/arch/x86/include/asm/paravirt.h
>>> @@ -735,13 +735,13 @@ extern void default_banner(void);
>>>   #else  /* __ASSEMBLY__ */
>>> -#define _PVSITE(ptype, ops, word, algn)		\
>>> +#define _PVSITE(ptype, ops, algn)		\
>>
>> Would you please drop the algn parameter, too? It isn't needed anymore
>> as the alignment can be hard coded to be 4 now. This would need to be
>> adjusted in the _paravirt_alt() macro, too.
>>
> OK, since the aligment is 4 bytes now, it seems that _ASM_ALIGN could
> be dropped too?

That's what I meant with the adjustment of _paravirt_alt().

I wouldn't drop _ASM_ALIGN, but replace it with ".align 4".


Juergen

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