Re: [PATCH 1/4] target/ppc: Code motion required to build disabling tcg

[David Gibson]

On Fri, Apr 23, 2021 at 10:28:14AM -0300, Fabiano Rosas wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > On Thu, Apr 22, 2021 at 04:35:34PM -0300, Fabiano Rosas wrote:
> >> Bruno Piazera Larsen <bruno.larsen@eldorado.org.br> writes:
> >> 
> >> >> > You are correct! I've just tweaked the code that defines spr_register 
> >> >> > and
> >> >> > it should be working now. I'm still working in splitting the SPR 
> >> >> > functions
> >> >> > from translate_init, since I think it would make it easier to prepare 
> >> >> > the
> >> >> > !TCG case and for adding new architectures in the future, and I found 
> >> >> > a
> >> >> > few more problems:
> >> >>
> >> >> Actually looking at the stuff below, I suspect that separating our
> >> >> "spr" logic specifically might be a bad idea.  At least some of the
> >> >> SPRs control pretty fundamental things about how the processor
> >> >> operates, and I suspect separating it from the main translation logic
> >> >> may be more trouble than it's worth.
> >> 
> >> I disagree with the code proximity argument. Having TCG code clearly
> >> separate from common code seems more important to me than having the SPR
> >> callbacks close to the init_proc functions.
> >
> > Hmm.. I may be misinterpreting what you're intending here.  I
> > certainly agree that separating TCG only code from common code is a
> > good idea.  My point, though, is that the vast majority of the SPR
> > code *is* TCG specific - there are just a relatively few cases where
> > SPRs have a common path.  That basically only happens when a) the SPR
> > can be affected by means other than the guest executing instructions
> > specifically to do that (i.e. usually by hypercalls) and b) accessing
> > the SPR has some side effects that need to be handled in both TCG and
> > KVM cases
> 
> The SPR code in translate_init.c.inc currently comprises of:
> 
> 1) the gen_spr* functions that are called during init_proc for each
> processor type;

Ah... that's one part of the confusion.  I forgot about these
functions.  These should indeed be common, despite sharing the gen_*()
prefix with mostly things that are explicitly TCG only.

> 2) the spr_register macros and _spr_register function that adds the SPRs
> to env->spr, called from (1);
> 
> 3) the TCG-specific SPR read|write callbacks, registered by (2);
> 
> 4) the KVM specific attribute one_reg_id, registered by (2).
> 
> The intention is to have one .c file (cpu_init.c) that deals with
> processor initialization, which is mostly setting PowerPCCPUClass
> attributes and registering the appropriate SPRs for each processor
> family (1,2). We're considering that to be shared between KVM and TCG
> for now.

Yes, that's what I'd expect.

> What is going into a separate file are the read and write SPR callbacks,
> which are TCG specific (3). They are still referenced from the other
> file when registering the SPRs, but are ignored when building for
> KVM-only. These are kept in a TCG-only compilation unit.

Ah, right, I'd forgotten that many of the callbacks are in
translate_init.c not translate.c.  Indeed, those will have to move.

> There's still a
> decision to be made whether we should have a separate spr_tcg file for
> them, or move them into translate.c along with the rest of TCG code.

Ah, I see.  Ok, yes, in that case moving them to a new TCG only spr
file makes more sense to me.  translate.c is already enormous.

> 
> The one_reg_id is just one attribute so that does not change.
> 
> > From the descriptions it sounded like you were trying to separate
> > *all* SPR code, not just these specific cases from the translation
> > core, and that's what I'm saying is a bad idea.
> 
> So, if anything, the SPR callbacks are moving _closer_ to the
> translation core.

Right.  Sorry for the misunderstanding.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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