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. 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 >> >