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Date:   Thu, 20 Jul 2023 08:52:16 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Petr Tesarik <petrtesarik@huaweicloud.com>
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Petr Tesarik <petr.tesarik.ext@huawei.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        James Seo <james@equiv.tech>,
        James Clark <james.clark@arm.com>,
        Kees Cook <keescook@chromium.org>,
        "moderated list:XEN HYPERVISOR ARM" <xen-devel@lists.xenproject.org>,
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        open list <linux-kernel@vger.kernel.org>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        "open list:XEN SWIOTLB SUBSYSTEM" <iommu@lists.linux.dev>,
        Roberto Sassu <roberto.sassu@huaweicloud.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>, petr@tesarici.cz
Subject: Re: [PATCH v4 0/8] Allow dynamic allocation of software IO TLB
 bounce buffers
Message-ID: <20230720065216.GB4395@lst.de>
References: <cover.1689261692.git.petr.tesarik.ext@huawei.com>
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Just to add a highlevel comment here after I feel like I need a little
more time to review the guts.

I'm still pretty concerned about the extra list that needs to be
consulted in is_swiotlb_buffer, but I can't really think of
anything better.  Maybe an xarray has better cache characteristics,
but that one requires even more allocations in the low-level dma map
path.

One thing I'd like to see for the next version is to make the
new growing code a config option at least for now.  It is a pretty
big change of the existing swiotlb behavior, and I want people to opt
into it conciously.  Maybe we can drop the option again after a few
years once everything has settled.