linux-kernel - Re: [PATCH v9 44/44] kselftest/arm64: Check GCR

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Date:   Thu, 12 Nov 2020 17:09:18 +0100
From:   Marco Elver <elver@...gle.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 44/44] kselftest/arm64: Check GCR_EL1 after context switch

On Thu, 12 Nov 2020 at 16:59, Alexander Potapenko <glider@...gle.com> wrote:
>
> On Tue, Nov 10, 2020 at 11:12 PM Andrey Konovalov <andreyknvl@...gle.com> wrote:
> >
> > From: Vincenzo Frascino <vincenzo.frascino@....com>
> >
> > This test is specific to MTE and verifies that the GCR_EL1 register
> > is context switched correctly.
> >
> > It spawn 1024 processes and each process spawns 5 threads. Each thread
>
> Nit: "spawns"
>
>
> > +       srand(time(NULL) ^ (pid << 16) ^ (tid << 16));
> > +
> > +       prctl_tag_mask = rand() % 0xffff;
>
> Nit: if you want values between 0 and 0xffff you probably want to use
> bitwise AND.

Another question would be, is the max here meant to be 0xffff or
0xffff-1. Because, as-is now, it's 0xffff-1. Only one of them has a
trivial conversion to bitwise AND ( x % 2^n == x & (2^n - 1) ).