Re: [PATCH] target/arm: do not use cc->do_interrupt for KVM directly

[Eduardo Habkost]

On Mon, Dec 07, 2020 at 10:06:55PM +0100, Claudio Fontana wrote:
> On 12/7/20 9:56 PM, Peter Maydell wrote:
> > On Mon, 7 Dec 2020 at 18:28, Eduardo Habkost <ehabkost@redhat.com> 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)

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.


> 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
> 
> 
> 
> 
> 

-- 
Eduardo