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

[Claudio Fontana]

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 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?

Ciao,

C

> 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
>>>
>>
>