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

[Claudio Fontana]

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.
Or where you thinking of ways to refactor current kvm code to use methods in 
CPUClass similarly to what Tcg does?

Ciao,

Claudio


> 
>>
>>> 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
>>
>