On 12/7/20 9:56 PM, Peter Maydell wrote: > On Mon, 7 Dec 2020 at 18:28, Eduardo Habkost <[email protected]> wrote: >> All signs seem to indicate that CPUClass.do_interrupt is >> TCG-specific, except for those two lines of code in >> target/arm/kvm64.c. The point of this patch would be to allow us >> to separate TCG-specific code from accel-independent code later. > > So it's TCG-specific except that we call it from KVM. > That doesn't sound very TCG-specific :-) > >> Maybe those signs are misleading us, and CPUClass.do_interrupt >> shouldn't be TCG-specific. If that's the case, why arm is the >> only architecture that uses CPUClass.do_interrupt outside >> TCG-specific code? > > So, the purpose of the do_interrupt method is "set the guest > CPU state up in the way that the architecture specifies > happens when an interrupt is taken" (set the program counter, > set things like the syndrome register that says what type > of exception happens, etc, etc). For TCG we obviously need > to do this for every interrupt/exception that happens. > For KVM, in most cases the kernel is responsible for > delivering an exception to the guest, because it's the > kernel that determines that it needs to do this. > The two oddball cases[*] in target/arm are for situations > where it is userspace code that has identified that it > needs to deliver an exception to the guest. The kernel's > KVM API doesn't provide a "please deliver an exception to > the guest" function, on the grounds that userspace could > do the work itself. So we need to do that work (setting the > PC, setting syndrome register, etc, etc). In theory we > could have a special version of the function for KVM > CPUs only, but since in fact the same code works just > fine in KVM and TCG we reuse it. > > I know that the macOS Hypervisor.Framework APIs are rather > lower-level than KVM (they do less work in the kernel and > push more of it onto userspace); it's possible that there > we might find more situations where userspace needs to do > "make the guest CPU take an exception"; I haven't investigated. > > [*] The two special cases are: > (1) the vcpu thread got a SIGBUS indicating a memory error, > and we need to deliver a synchronous external abort > exception to the guest to let it know about the error > (2) the kernel told us about a debug exception (breakpoint, > watchpoint, etc) but it turns out not to be for one of > QEMU's own gdbstub breakpoints/watchpoints, so it > must be one the guest itself has set up, and so we need > to deliver it to the guest > > These are fairly obscure, and it wouldn't surprise me if > other target archs had picked a different separation of > concerns between userspace and the kernel such that userspace > didn't need to manually deliver an exception. > > thanks > -- PMM >
Hello Peter, thank you for the explanation, interesting read. As I understand it, for the purpose of code separation, we could: 1) skip do_interrupt move to the separate tcg_ops structure, wait until KVM/ARM uses another approach (if ever) 2) do the move, and just call arm_cpu_do_interrupt() directly, since for KVM64 it is the only one that can be assigned to cc->do_interrupt(). Which way would you suggest? Thanks, Claudio
