linux-kernel - Re: [PATCH v4 0/8] Allow dynamic allocation of software IO TLB bounce buffers

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Message-ID: <20230720101320.72bbd2fc@meshulam.tesarici.cz>
Date:   Thu, 20 Jul 2023 10:13:20 +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 0/8] Allow dynamic allocation of software IO TLB
 bounce buffers

On Thu, 20 Jul 2023 08:52:16 +0200
Christoph Hellwig <hch@....de> wrote:

> 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.

Fine with me. I removed it after all my testing showed no performance
impact as long as the size of the initial SWIOTLB is kept at the
default value (and sufficient for the workload), but it's OK for me if
dynamic SWIOTLB allocations are off by default.

OTOH I'd like to make it a boot-time option rather than build-time
option. Would that be OK for you?

Petr T