On Tue, Dec 08, 2020 at 03:34:03PM +0100, 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 <ehabk...@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.
So, methods that have accel-specific implementations, which is exactly why we have the CpusAccel struct (renamed to AccelCpuClass in Claudio's cleanup series). Is there any reason to not move CPUClass.do_interrupt to AccelCpuClass.do_interrupt? -- Eduardo