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 <[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 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
