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 <[email protected]> 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 >>> >> >
