linux-kernel - Re: [PATCH v1 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

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Date:   Thu, 9 Jul 2020 17:14:28 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     <catalin.marinas@....com>, <will@...nel.org>,
        <suzuki.poulose@....com>, <maz@...nel.org>, <steven.price@....com>,
        <guohanjun@...wei.com>, <olof@...om.net>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <arm@...nel.org>, <xiexiangyou@...wei.com>,
        <prime.zeng@...ilicon.com>, <zhangshaokun@...ilicon.com>,
        <kuhn.chenqun@...wei.com>
Subject: Re: [PATCH v1 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

On 2020/7/9 17:10, Zhenyu Ye wrote:
> +	/*
> +	 * When cpu does not support TLBI RANGE feature, we flush the tlb
> +	 * entries one by one at the granularity of 'stride'.
> +	 * When cpu supports the TLBI RANGE feature, then:
> +	 * 1. If pages is odd, flush the first page through non-RANGE
> +	 *    instruction;
> +	 * 2. For remaining pages: The minimum range granularity is decided
> +	 *    by 'scale', so we can not flush all pages by one instruction
> +	 *    in some cases.
> +	 *
> +	 * For example, when the pages = 0xe81a, let's start 'scale' from
> +	 * maximum, and find right 'num' for each 'scale':
> +	 *
> +	 *  When scale = 3, we can flush no pages because the minumum
> +	 * range is 2^(5*3 + 1) = 0x10000.
> +	 *  When scale = 2, the minimum range is 2^(5*2 + 1) = 0x800, we can
> +	 * flush 0xe800 pages this time, the num = 0xe800/0x800 - 1 = 0x1c.
> +	 * Remain pages is 0x1a;
> +	 *  When scale = 1, the minimum range is 2^(5*1 + 1) = 0x40, no page
> +	 * can be flushed.
> +	 *  When scale = 0, we flush the remaining 0x1a pages, the num =
> +	 * 0x1a/0x2 - 1 = 0xd.
> +	 *
> +	 * However, in most scenarios, the pages = 1 when flush_tlb_range() is
> +	 * called. Start from scale = 3 or other proper value (such as scale =
> +	 * ilog2(pages)), will incur extra overhead.
> +	 * So increase 'scale' from 0 to maximum, the flush order is exactly
> +	 * opposite to the example.
> +	 */

The comments may be too long, probably should be moved to commit messages.

Thanks,
Zhenyu