linux-kernel - Re: [PATCH 1/6] kselftest/arm64: Add utilities and a test to validate mte memory

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Date:   Mon, 21 Sep 2020 15:18:19 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>, Will Deacon <will@...nel.org>,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Gabor Kertesz <gabor.kertesz@....com>
Subject: Re: [PATCH 1/6] kselftest/arm64: Add utilities and a test to
 validate mte memory

On Tue, Sep 01, 2020 at 02:57:14PM +0530, Amit Daniel Kachhap wrote:
> diff --git a/tools/testing/selftests/arm64/mte/mte_helper.S b/tools/testing/selftests/arm64/mte/mte_helper.S
> new file mode 100644
> index 000000000000..91af6d1293f8
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/mte/mte_helper.S
> @@ -0,0 +1,116 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2020 ARM Limited */
> +
> +#include "mte_def.h"
> +
> +#define ENTRY(name) \
> +	.globl name ;\
> +	.p2align 2;\
> +	.type name, @function ;\
> +name:
> +
> +#define ENDPROC(name) \
> +	.size name, .-name ;
> +
> +	.text
> +/*
> + * mte_insert_random_tag: Insert random tag and different from
> + *			 the orginal tag if source pointer has it.
> + * Input:
> + *		x0 - source pointer with a tag/no-tag
> + * Return:
> + *		x0 - pointer with random tag
> + */
> +ENTRY(mte_insert_random_tag)
> +	mov	x1, #0x0
> +	gmi	x1, x0, x1
> +	irg	x0, x0, x1
> +	ret
> +ENDPROC(mte_insert_random_tag)

What was the reason for gmi here? The test fails when you have an
include mask of 0x8000 (exclude mask 0x7fff) and x0 has tag 0xf. In this
case we exclude the only allowed tag here, so the CPU falls back to the
default tag 0.

You can (a) stop the check_multiple_included_tags() earlier to have two
allowed tags here, (b) clear the pointer old tag so that you don't end
up in this scenario or (c) simply remove the gmi. My preference is the
latter, we don't test the hardware here, we only want to check whether
the kernel sets the GCR_EL1 correctly.

BTW, you also remove mov x1, #0, just:

	irg	x0, x0, xzr

-- 
Catalin