Re: [PATCH v3 16/34] xen/lib: introduce generic find next bit operations

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Tue, 2024-01-23 at 14:37 +0100, Jan Beulich wrote:
> On 23.01.2024 13:34, Oleksii wrote:
> > On Tue, 2024-01-23 at 12:14 +0100, Jan Beulich wrote:
> > > On 22.12.2023 16:13, Oleksii Kurochko wrote:
> > > > --- a/xen/common/Kconfig
> > > > +++ b/xen/common/Kconfig
> > > > @@ -47,6 +47,9 @@ config ARCH_MAP_DOMAIN_PAGE
> > > >  config GENERIC_BUG_FRAME
> > > >         bool
> > > >  
> > > > +config GENERIC_FIND_NEXT_BIT
> > > > +       bool
> > > 
> > > There's no need for this, as ...
> > > 
> > > > --- a/xen/lib/Makefile
> > > > +++ b/xen/lib/Makefile
> > > > @@ -3,6 +3,7 @@ obj-$(CONFIG_X86) += x86/
> > > >  lib-y += bsearch.o
> > > >  lib-y += ctors.o
> > > >  lib-y += ctype.o
> > > > +lib-$(CONFIG_GENERIC_FIND_NEXT_BIT) += find-next-bit.o
> > > 
> > > ... you're moving this to lib/. Or have you encountered any issue
> > > with building this uniformly, and you forgot to mention this in
> > > the description?
> > I didn't check. My intention was to provide opportunity to check if
> > an
> > architecture want to use generic version or not. Otherwise, I
> > expected
> > that we will have multiple definiotion of the funcion.
> > 
> > But considering that they are all defined under #ifdef...#endif we
> > can
> > remove the declaration of the config GENERIC_FIND_NEXT_BIT.
> 
> What #ifdef / #endif would matter here? Whats in lib/ is intended to
> be
> generic anyway. And what is in the resulting lib.a won't be used by
> an
> arch if it has an arch-specific implementation. 
If what is implemented in lib.a won't be used by an arch if it has an
arch-specific implementation then, for sure, I have to drop
CONFIG_GENERIC_FIND_NEXT_BIT.
But I am not really understand if lib.a is linked with Xen, then it
should be an issue then if some arch implement find-next-bit function
we will have to multiple definitions ( one in lib.a and one arch
specific ). Probably, I have to look at how it is done.

> Problems could arise if
> an arch had an inline function colliding with the out-of-line one.
> But
> that's about the old case where I could see a need to make the
> building
> of one of the objects conditional. And you'll note that withing this
> Makefile there are pretty few conditionals.
Could you please clarify What does it mean "out-of-line" ?
> 
> > > > --- /dev/null
> > > > +++ b/xen/lib/find-next-bit.c
> > > > [...]
> > > 
> > > I was going to ask that you convince git to actually present a
> > > proper
> > > diff, to make visible what changes. But other than the
> > > description
> > > says
> > > you don't really move the file, you copy it. Judging from further
> > > titles
> > > there's also nowhere you'd make Arm actually use this now generic
> > > code.
> > I wanted to do it separately, outside this patch series to simplify
> > review and not have Arm specific changes in RISC-V patch series.
> 
> Then do it the other way around: Make a separate _prereq_ change
> truly
> moving the file.
So this one patch should be separated by 2? One which moves find-next-
bit.c from Arm to xen/lib, and second where xen/lib/Makefile is
updated.

> 
> > Regarding a proper diff, you would like me to make git shows that
> > it
> > was copy from Arm and it is not newly created file. Am I understand
> > you
> > correctly?
> 
> Not quite, I think. Git has move detection (and we've seen that in
> action in other patches of yours). So when truly moving a file, what
> (if anything) is changed is easily visible.
I think I am still a little bit confused. But I think the answer on my
question above can clarify that.

~ Oleksii