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

[Philippe Mathieu-Daudé]

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.

> 
> Ciao,
> 
> C