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

[Juergen Gross via Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

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@xxxxxxxxxxxx>
> > > ---
> > >   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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