linux-kernel - Re: [PATCH v4 11/11] KVM: selftests: Enable tunning of err_margin

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Date: Thu, 21 Dec 2023 10:58:40 +0800
From: Haibo Xu <xiaobo55x@...il.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Haibo Xu <haibo1.xu@...el.com>, ajones@...tanamicro.com, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Paolo Bonzini <pbonzini@...hat.com>, 
	Shuah Khan <shuah@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	James Morse <james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Zenghui Yu <yuzenghui@...wei.com>, Anup Patel <anup@...infault.org>, 
	Atish Patra <atishp@...shpatra.org>, Guo Ren <guoren@...nel.org>, 
	Mayuresh Chitale <mchitale@...tanamicro.com>, Greentime Hu <greentime.hu@...ive.com>, 
	wchen <waylingii@...il.com>, Conor Dooley <conor.dooley@...rochip.com>, 
	Heiko Stuebner <heiko@...ech.de>, Minda Chen <minda.chen@...rfivetech.com>, 
	Samuel Holland <samuel@...lland.org>, Jisheng Zhang <jszhang@...nel.org>, 
	Sean Christopherson <seanjc@...gle.com>, Peter Xu <peterx@...hat.com>, Like Xu <likexu@...cent.com>, 
	Vipin Sharma <vipinsh@...gle.com>, 
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Aaron Lewis <aaronlewis@...gle.com>, 
	Thomas Huth <thuth@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, kvm-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 11/11] KVM: selftests: Enable tunning of err_margin_us
 in arch timer test

On Wed, Dec 20, 2023 at 9:58 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On Wed, 20 Dec 2023 13:51:24 +0000,
> Haibo Xu <xiaobo55x@...il.com> wrote:
> >
> > On Wed, Dec 20, 2023 at 5:00 PM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > On 2023-12-20 06:50, Haibo Xu wrote:
> > > > On Wed, Dec 20, 2023 at 2:22 AM Marc Zyngier <maz@...nel.org> wrote:
> > > >>
> > > >> On Tue, 12 Dec 2023 09:31:20 +0000,
> > > >> Haibo Xu <haibo1.xu@...el.com> wrote:
> > > >> > diff --git a/tools/testing/selftests/kvm/include/timer_test.h b/tools/testing/selftests/kvm/include/timer_test.h
> > > >> > index 968257b893a7..b1d405e7157d 100644
> > > >> > --- a/tools/testing/selftests/kvm/include/timer_test.h
> > > >> > +++ b/tools/testing/selftests/kvm/include/timer_test.h
> > > >> > @@ -22,6 +22,7 @@ struct test_args {
> > > >> >       int nr_iter;
> > > >> >       int timer_period_ms;
> > > >> >       int migration_freq_ms;
> > > >> > +     int timer_err_margin_us;
> > > >>
> > > >> ... except that you are storing it as a signed value. Some consistency
> > > >> wouldn't hurt, really, and would avoid issues when passing large
> > > >> values.
> > > >>
> > > >
> > > > Yes, it's more proper to use an unsigned int for the non-negative error
> > > > margin.
> > > > Storing as signed here is just to keep the type consistent with that
> > > > of timer_period_ms
> > > > since there will be '+' operation in other places.
> > > >
> > > >         tools/testing/selftests/kvm/aarch64/arch_timer.c
> > > >         /* Setup a timeout for the interrupt to arrive */
> > > >          udelay(msecs_to_usecs(test_args.timer_period_ms) +
> > > >              test_args.timer_err_margin_us);
> > >
> > > But that's exactly why using a signed quantity is wrong.
> > > What does it mean to have a huge *negative* margin?
> > >
> >
> > Hi Marc,
> >
> > I agree that negative values are meaningless for the margin.
> > If I understand correctly, the negative margin should be filtered by
> > assertion in atoi_non_negative().
>
> No. Please.
>
> atoi_non_negative() returns a uint32_t, which is what it should do.
> The bug is squarely in the use of an 'int' to store such value, and it
> is the *storage* that turns a positive value into a negative one.
>

Thanks for the detailed info!

May I understand that your concern is mainly for a platform with 64bit int type,
which may trigger the positive to negative convert?

If so, I think we may need to do a clean up for the test code since
several other
places have the same issue.

Regards,
Haibo

>         M.
>
> --
> Without deviation from the norm, progress is not possible.