Re: [PATCH] KVM: arm64: Ratelimit error log during guest debug exception

[Marc Zyngier]

On 2021-08-23 19:13, Raghavendra Rao Ananta wrote:
> On Sat, Aug 21, 2021 at 3:56 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > On Sat, 21 Aug 2021 00:01:24 +0100,
> > Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> > >
> > > [1  <text/plain; UTF-8 (7bit)>]
> > > On Fri, Aug 20, 2021 at 2:29 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, 19 Aug 2021 23:34:06 +0100,
> > > > Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> > > > >
> > > > > Potentially, the guests could trigger a debug exception that's
> > > > > outside the exception class range.
> > > >
> > > > How? All the exception classes that lead to this functions are already
> > > > handled in the switch/case statement.
> > > >
> > > I guess I didn't think this through. Landing into kvm_handle_guest_debug()
> > > itself is not possible :)
> >
> > Exactly.
> >
> > > > My take on this is that this code isn't reachable, and that it could
> > > > be better rewritten as:
> > > >
> > > > diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> > > > index 6f48336b1d86..ae7ec086827b 100644
> > > > --- a/arch/arm64/kvm/handle_exit.c
> > > > +++ b/arch/arm64/kvm/handle_exit.c
> > > > @@ -119,28 +119,14 @@ static int kvm_handle_guest_debug(struct kvm_vcpu
> > > *vcpu)
> > > >  {
> > > >         struct kvm_run *run = vcpu->run;
> > > >         u32 esr = kvm_vcpu_get_esr(vcpu);
> > > > -       int ret = 0;
> > > >
> > > >         run->exit_reason = KVM_EXIT_DEBUG;
> > > >         run->debug.arch.hsr = esr;
> > > >
> > > > -       switch (ESR_ELx_EC(esr)) {
> > > > -       case ESR_ELx_EC_WATCHPT_LOW:
> > > > +       if (ESR_ELx_EC(esr) ==  ESR_ELx_EC_WATCHPT_LOW)
> > > >                 run->debug.arch.far = vcpu->arch.fault.far_el2;
> > > > -               fallthrough;
> > > > -       case ESR_ELx_EC_SOFTSTP_LOW:
> > > > -       case ESR_ELx_EC_BREAKPT_LOW:
> > > > -       case ESR_ELx_EC_BKPT32:
> > > > -       case ESR_ELx_EC_BRK64:
> > > > -               break;
> > > > -       default:
> > > > -               kvm_err("%s: un-handled case esr: %#08x\n",
> > > > -                       __func__, (unsigned int) esr);
> > > > -               ret = -1;
> > > > -               break;
> > > > -       }
> > > >
> > > > -       return ret;
> > > > +       return 0;
> > > >  }
> > > >
> > > This looks better, but do you think we would be compromising on readability?
> >
> > I don't think so. The exit handler table is, on its own, pretty
> > explicit about what we route to this handler, and the comment above
> > the function clearly states that we exit to userspace for all the
> > debug ECs.
>
> Sounds great. I'm happy to send out a patch with you as 'Suggested-by' 
> , if you
> are okay with it.

Fire away!

        M.
--
Jazz is not dead. It just smells funny...