Re: [PATCH] target/arm: do not use cc->do_interrupt for KVM directly

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Dec 08, 2020 at 03:34:03PM +0100, Philippe Mathieu-Daudé wrote:
> On 12/8/20 2:55 PM, Claudio Fontana wrote:
> > On 12/8/20 2:51 PM, Claudio Fontana wrote:
> >> On 12/8/20 2:27 PM, Philippe Mathieu-Daudé wrote:
> >>> On 12/7/20 10:50 PM, Peter Maydell wrote:
> >>>> On Mon, 7 Dec 2020 at 21:26, Eduardo Habkost <ehabkost@redhat.com> wrote:
> >>>>> My understanding is that there's no reason for ARM KVM to use
> >>>>> another approach, and that CPUClass.do_interrupt is not really
> >>>>> TCG-specific.
> >>>>>
> >>>>> Do we have any case where the CPUClass.do_interrupt
> >>>>> implementation is really TCG-specific, or it is just a
> >>>>> coincidence that most other accelerators simply don't to call the
> >>>>> method?  It looks like the only cases where the
> >>>>> CPUClass.do_interrupt assignment is conditional on CONFIG_TCG are
> >>>>> i386 and s390x.
> >>>>
> >>>> Looking at PPC, its kvm_handle_debug() function does a
> >>>> direct call to ppc_cpu_do_interrupt(). So the code of
> >>>> its do_interrupt method must be ok-for-KVM, it's just that
> >>>> it doesn't use the method pointer. (It's doing the same thing
> >>>> Arm is -- if a debug event turns out not to be for QEMU itself,
> >>>> inject a suitable exception into the guest.)
> >>>>
> >>>> So of our 5 KVM-supporting architectures:
> >>>>
> >>>>  * i386 and s390x have kernel APIs for "inject suitable
> >>>>    exception", don't need to call do_interrupt, and make
> >>>>    the cc->do_interrupt assignment only ifdef CONFIG_TCG,
> >>>>    so that the code for do_interrupt need not be compiled
> >>>>    into a KVM-only binary. (In both cases the code for the
> >>>>    function is in a source file that the meson.build puts
> >>>>    into the source list only if CONFIG_TCG)
> >>>>  * ppc and arm both need to use this code even in a KVM
> >>>>    only binary. Neither of them #ifdef the cc->do_interrupt
> >>>>    assignment, because there's not much point at the moment
> >>>>    if you're not going to try to compile out the code.
> >>>>    ppc happens to do a direct function call, and arm happens
> >>>>    to go via the cc->do_interrupt pointer, but I don't
> >>>>    think there's much significance in the choice either way.
> >>>>    In both cases, the only places making the call are within
> >>>>    architecture-specific KVM code.
> >>>>  * mips KVM does neither of these things, probably because it is
> >>>>    not sufficiently featureful to have run into the cases
> >>>>    where you might want to re-inject an exception and it's
> >>>>    not being sufficiently used in production for anybody to
> >>>>    have looked at minimising the amount of code in a
> >>>>    KVM-only QEMU binary for it.
> >>>>
> >>>> So in conclusion we have a basically 50:50 split between
> >>>> "use the same do_interrupt code as TCG" and "have a kernel
> >>>> API to make the kernel do the work", plus one arch that
> >>>> probably hasn't had to make the choice yet.   ¯\_(ツ)_/¯
> >>>
> >>> Why not introduce KVMCpuOperations similar to TCGCpuOperations
> >>> Claudio is introducing, and declare the do_interrupt(CPUState*)
> >>> in both structures?
> >>>
> >>> Then we can assign the same handler to both fields, TCG keeps
> >>> calling cc->tcg->do_interrupt(), KVM calls cc->kvm->do_interrupt().
> >>> This allow building with a particular accelerator, while staying
> >>> compliant with the current 50:50 split...
> >>
> >>
> >> Hi Philippe,
> >>
> >> in principle interesting, but KVMCpuOperations would end up currently containing do_interrupt only..
> >> seems a bit overkill for just one method.
> 
> I don't see this being a problem, if this makes code clearer
> (think about maintainability).
> 
> > I mean, all the others in CPUClass are common between TCG and KVM, I don't see a lot that is KVM-only there that would warrant a KVMCPUOps structure
> > 
> >> Or where you thinking of ways to refactor current kvm code to use methods in CPUClass similarly to what Tcg does?
> >>
> > 
> > But maybe this is where you were going with this?
> 
> No, not really. I'm looking for a design to enforce correctness,
> while keeping the 2 choices Peter mentioned available.
> 
> - "use the same do_interrupt code as TCG":
> 
> cc->tcg.do_interrupt = x86_cpu_do_interrupt;
> cc->kvm.do_interrupt = NULL;
> 
> cc->tcg.do_interrupt = s390_cpu_do_interrupt;
> cc->kvm.do_interrupt = NULL;
> 
> - "have a kernel API to make the kernel do the work"
> 
> cc->tcg.do_interrupt = arm_cpu_do_interrupt;
> cc->kvm.do_interrupt = arm_cpu_do_interrupt;
> 
> cc->tcg.do_interrupt = ppc_cpu_do_interrupt;
> cc->kvm.do_interrupt = ppc_cpu_do_interrupt;
> 
> Looks easy to review, hard to misplace #ifdef'ry.

So, methods that have accel-specific implementations, which is
exactly why we have the CpusAccel struct (renamed to
AccelCpuClass in Claudio's cleanup series).

Is there any reason to not move CPUClass.do_interrupt to
AccelCpuClass.do_interrupt?

-- 
Eduardo