Re: [PATCH kvm-unit-tests 2/3] arm64: timer: Fix test on APM X-Gene

Andrew Jones
2017-07-18T12:15:27Z

On Tue, Jul 18, 2017 at 12:35:12PM +0200, Christoffer Dall wrote:
> On Tue, Jul 18, 2017 at 12:05:03PM +0200, Andrew Jones wrote:
> > On Fri, Jul 14, 2017 at 08:45:12AM -0700, Christoffer Dall wrote:
> > > On Fri, Jul 14, 2017 at 09:04:10AM +0100, Marc Zyngier wrote:
> > > > On 13/07/17 20:20, Christoffer Dall wrote:
> > > > > When running the vtimer test on an APM X-Gene, setting the timer value
> > > > > to (2^64 - 1) apparently results in the timer always firing, even
> > > > > thought the counter is mich lower than the cval.
> > > >
> > > > Note that the system counter is only guaranteed to be at least 56 bit
> > > > wide (see DDI0487B.a G5.1.2), and I seem to remember that X-Gene only
> > > > has the minimum. This could explain why setting the comparator to a
> > > > value greater than (2^56 - 1) leads to a firing timer (the comparator
> > > > appears to be in the past).
> >
> > Ah, that explains why when I tried setting it to ~0 on my mustang, and
> > then reading it back, it was always 2^56 - 1 instead. However my mustang
> > still also requires me to clear bit 31, otherwise the vcpu hangs.
> >
> > >
> > > Thanks for pointing that out, that makes good sense. So then we should
> > > definitely fix the test.
> > >
> > > We could either set it to 2^56 - 1 instead, or just keep the 10s as used
> > > in this patch, because the whole test times out after 2s anyway.
> >
> > With the 10s version the test runs and passes on my mustang, so on one
> > hand I prefer it. OTOH, testing to the spec, by using 2^56 - 1, seems
> > more correct and allows one to find issues like the one I have on my
> > mustang, i.e. a vcpu hang when bit 31 isn't clear.
> >
> > I guess I lean more towards testings to the spec, but not enough to
> > ask for a v2 of the patch. It's up to you.
>
> I think we should have something that tests KVM on the platforms we have
> and that are available for people's use. I don't think we should verify
> the architecture as much. People use the m400 (basically
> Mustang) in CloudLab, for example, which is we I keep caring about that.
>
> I think this test was designed to test "if I program a timer to some
> time in the future it shouldn't fire right away", which is still what
> we test with this patch.
>
> If we want to add a "the platform provides a timer with 56 valid bits in
> the counter and compare register", then I think it should be a separate
> test, and the the user can see that "basic stuff works", "architecture
> compliance not so much" and shrug accordingly.

Two separate tests sounds good to me. Although, if the value of 'now' is
large enough that now + 10s will set bit 31, then a mustang run (at least
mine) will fail - and most likely cause a lot of confusion, since it
normally does not. We should probably attempt to address that known issue
in some way as well.

Thanks,
drew