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)_/¯ > 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
