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

Christoffer Dall
2017-07-18T10:35:12Z

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.

Thanks,
-Christoffer