Re: [PATCH] KVM: SVM: fix task switch emulation on INTn instruction.

Sean Christopherson
2022-07-15T00:00:57Z

On Thu, Jul 14, 2022, Jim Mattson wrote:
> On Thu, Jul 14, 2022 at 4:24 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Fri, Jul 15, 2022, Maciej S. Szmigiero wrote:
> > > On 14.07.2022 15:57, Maxim Levitsky wrote:
> > > > On Thu, 2022-07-14 at 15:50 +0200, Maciej S. Szmigiero wrote:
> > > > > On 14.07.2022 14:44, Maxim Levitsky wrote:
> > > > > > Recently KVM's SVM code switched to re-injecting software interrupt events,
> > > > > > if something prevented their delivery.
> > > > > >
> > > > > > Task switch due to task gate in the IDT, however is an exception
> > > > > > to this rule, because in this case, INTn instruction causes
> > > > > > a task switch intercept and its emulation completes the INTn
> > > > > > emulation as well.
> > > > > >
> > > > > > Add a missing case to task_switch_interception for that.
> > > > > >
> > > > > > This fixes 32 bit kvm unit test taskswitch2.
> > > > > >
> > > > > > Fixes: 7e5b5ef8dca322 ("KVM: SVM: Re-inject INTn instead of retrying the insn on "failure"")
> > > > > >
> > > > > > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > > > > > ---
> > > > >
> > > > > That's a good catch, your patch looks totally sensible to me.
> > > > > People running Win 3.x or OS/2 on top of KVM will surely be grateful for it :)
> > > >
> > > > Yes and also people who run 32 bit kvm unit tests :)
> > >
> > > It looks like more people need to do this regularly :)
> >
> > I do run KUT on 32-bit KVM, but until I hadn't done so on AMD for a long time and
> > so didn't realize the taskswitch2 failure was a regression. My goal/hope is to
> > we'll get to a state where we're able to run the full gamut of tests before things
> > hit kvm/queue, but the number of permutations of configs and module params means
> > that's easier said than done.
> >
> > Honestly, it'd be a waste of people's time to expect anyone else beyond us few
> > (and CI if we can get there) to test 32-bit KVM. We do want to keep it healthy
> > for a variety of reasons, but I'm quite convinced that outside of us developers,
> > there's literally no one running 32-bit KVM.
>
> It shouldn't be necessary to run 32-bit KVM to run 32-bit guests! Or
> am I not understanding the issue that was fixed here?

Ah, no, I'm the one off in the weeds. I only ever run 32-bit KUT on 32-bit VMs
because I've been too lazy to "cross"-compile. Time to remedy that...