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

[Peter Maydell]

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)_/¯

> Oh, I thought you were arguing that CPUClass.do_interrupt is
> not TCG_specific.

Well, I don't think it really is TCG-specific. But as
a pragmatic thing, if these two lines in the Arm code
are getting in the way of stopping us from having a
useful compile-time check that code that's not supposed
to call this method isn't calling it, I think the balance
maybe leans towards just making the direct function call.
I guess it depends whether you think people are likely to
accidentally make cc->do_interrupt calls in non-target-specific
code that gets used by KVM (which currently would crash if that
code path is exercised on x86 or s390x, but under the
proposed change would become a compile error).

thanks
-- PMM