linux-kernel - Re: [PATCH v4 8/8] swiotlb: search the software IO TLB only if a device makes use of it

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Message-ID: <20230720100238.58f11c06@meshulam.tesarici.cz>
Date:   Thu, 20 Jul 2023 10:02:38 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Christoph Hellwig <hch@....de>
Cc:     Petr Tesarik <petrtesarik@...weicloud.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Petr Tesarik <petr.tesarik.ext@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        James Seo <james@...iv.tech>,
        James Clark <james.clark@....com>,
        Kees Cook <keescook@...omium.org>,
        "moderated list:XEN HYPERVISOR ARM" <xen-devel@...ts.xenproject.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "open list:XEN SWIOTLB SUBSYSTEM" <iommu@...ts.linux.dev>,
        Roberto Sassu <roberto.sassu@...weicloud.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH v4 8/8] swiotlb: search the software IO TLB only if a
 device makes use of it

On Thu, 20 Jul 2023 08:47:44 +0200
Christoph Hellwig <hch@....de> wrote:

> Any reason this can't just do a list_empty_careful on the list
> instead of adding yet another field that grows struct device?

On which list?

The dma_io_tlb_pools list only contains transient pools, but a device
may use bounce buffers from a regular pool.

The dma_io_tlb_mem.pools list will always be non-empty, unless the
system runs without SWIOTLB.

On a system which does have a SWIOTLB, the flag allows to differentiate
between devices that actually use bounce buffers and devices that do
not (e.g. because they do not have any addressing limitations).

Petr T