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

[Claudio Fontana]

On 12/8/20 3:34 PM, 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.

to limit feature creep in the series even less error prone would be to put 
do_interrupt only for #tcg for now,
I think, and call the function directly in the only place it is necessary 
(arm/kvm64.c),
and then revisit this when we do the actual code split between tcg and kvm (on 
arm there are already quite a few things to do I think).

How does it sound?

Ciao,

Claudio