Re: [PATCH v2 1/5] x86/livepatch: set function alignment to ensure minimal function size

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Dec 05, 2023 at 01:42:42PM +0000, Andrew Cooper wrote:
> On 28/11/2023 10:03 am, Roger Pau Monne wrote:
> > diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
> > index f3abdf9cd111..f629157086d0 100644
> > --- a/xen/arch/x86/Makefile
> > +++ b/xen/arch/x86/Makefile
> > @@ -82,6 +82,8 @@ obj-$(CONFIG_COMPAT) += x86_64/platform_hypercall.o
> >  obj-y += sysctl.o
> >  endif
> >  
> > +CFLAGS-$(CONFIG_LIVEPATCH) += 
> > -falign-functions=$(CONFIG_CC_FUNCTION_ALIGNMENT)
> 
> I'd really prefer not to express it like this.  For one, a major reason
> for using an alignment of 16b or more is simply performance.
> 
> Also, it isn't "CC" when we get the asm macros working.
> 
> Copy Linux more closely.  Then, you have LIVEPATCH select
> FUNCTION_ALIGNMENT_{8,16}B as appropriate.  And PERFORMANCE selects
> FUNCTION_ALIGNMENT_16B or perhaps 32B depending on uarch.

So just use CONFIG_FUNCTION_ALIGNMENT and drop the CC part of it?
That would indeed be fine.  We will also need to adjust
CC_SPLIT_SECTIONS to drop the CC_ prefix when we start using it in
assembly code.

> If we ever get around to having KCFI, then we need 16B irrespective of
> anything else.
> 
> 
> 
> As for the subject, it's not really about size; the function size is
> still going to be small irrespective of the alignment.

What about wording it like:

x86/livepatch: set function alignment to ensure minimal space between functions

Thanks, Roger.