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

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Date: Thu, 21 Dec 2023 09:25:02 +0000
From: Marc Zyngier <maz@...nel.org>
To: Haibo Xu <xiaobo55x@...il.com>
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 Thu, 21 Dec 2023 02:58:40 +0000,
Haibo Xu <xiaobo55x@...il.com> wrote:
> 
> 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?

No. It specifically applies to architectures with a 32bit int type,
which is... *EVERYTHING*. Here's a basic example:

<quote>
#include <stdio.h>

int main(int argc, char *argv[])
{
	int x = 1U << 31;

	printf("%d (%d)\n", x, sizeof(x));
	return 0;
}
</quote>

which returns "-2147483648 (4)" on any platform.

This really is basic C, and I am very worried that you don't see the
issue. I strongly suggest that you go and read about the C type system
before touching this code.

	M.

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