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Re: modular tcg

From: Gerd Hoffmann
Subject: Re: modular tcg
Date: Thu, 29 Jul 2021 12:26:27 +0200


> > It's basically two groups:
> > 
> >  * Arch-specific (functions taking CPUX86State as argument), most of the
> >    unresolved symbols in target/i386/ and i386/ directories go into this
> >    category.
> Yes, and we need to think about all targets, not just i386.

Sure.  I just want go forward in small steps, so my plan is to tackle
them one by one (starting with i386).

> >  * Common (functions taking CPUState as argument).  Everything else.
> > 
> > The common functions could be added TCGCPUOps to allow them being called via
> TCGCCPUOps are target-specific in their implementation, so I guess
> it's the arch specific part that could be TCGCPUOps (maybe, would need
> deep thinking).

Ok, lets make it three groups then.

  (1) generic interface, arch implementation (this is what we have
      TCGCPUOps hooks right now).
  (2) generic interface, generic implementation (functions taking a
      CPUState as argument, simliar to group (1).
  (3) arch-specific interface and implementation (functions taking a
      CPUX86State argument).

We could add group (2) to TCGCPUOps for this ...

> > CPUClass->tcg_ops->$name instead of a direct symbol reference.  Not sure 
> > this
> > is a good idea though.  Experimental patch covering tcg_exec_realizefn +
> > tcg_exec_unrealizefn below.

... but as I sayed, not sure this is the best plan.

Adding group (3) to TCGCPUOps is a non-starter IMHO given that the
function prototypes are arch-specific (using CPUX86State) and also
the interfaces actually needed are arch-specific.  something like
x86_register_ferr_irq or cpu_x86_update_dr7 simply doesn't exist on
!x86.  I guess we'll need TCG${arch}Ops for those.

> > No idea yet how to handle arch-specific bits best.  Seems there is no 
> > existing
> > infrastructure to untangle target-specific code and tcg, so this probably 
> > needs
> > something new.
> We need target-specific modules. They could at the beginning absorb
> also the non-target specific parts in my view.  So you have a big
> tcg-arm module, a tcg-i386 module etc.
> I think I sketched already the idea in the Makefile I shared before?

We have target-specific modules in master branch.
Used for qtest (all archs) and tcg (i386/x86_64 only, accel ops only).

The build system changes to build more tcg bits modular are here:
Doesn't build due unresolved symbols, but shows which code
changes/cleanups/reorganizations are needed for (more) modular tcg.

take care,

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