linux-kernel - Re: [PATCH v2 RESEND 4/7] swiotlb: Dynamically allocated bounce buffers

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Message-ID: <20230523135317.6b9e086f@meshulam.tesarici.cz>
Date:   Tue, 23 May 2023 13:53:17 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Christoph Hellwig <hch@....de>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Petr Tesarik <petrtesarik@...weicloud.com>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Kim Phillips <kim.phillips@....com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH v2 RESEND 4/7] swiotlb: Dynamically allocated bounce
 buffers

On Tue, 23 May 2023 10:54:11 +0100
Catalin Marinas <catalin.marinas@....com> wrote:

> On Wed, May 17, 2023 at 01:27:48PM +0200, Petr Tesařík wrote:
> > On Wed, 17 May 2023 08:35:10 +0200
> > Petr Tesařík <petr@...arici.cz> wrote:  
> > > Anyway, my greatest objection to allocating additional swiotlb chunks is
> > > that _all_ of them must be searched to determine that the physical
> > > address does _not_ belong to a swiotlb, incurring performance penalty  
> > 
> > I thought about this part again, and I overlooked one option. We can
> > track only the _active_ swiotlbs for each device. If a device never
> > needs a swiotlb, there is no active swiotlb, and is_swiotlb_buffer()
> > short-circuits to false. This should avoid all collateral damage to
> > innocent devices.  
> 
> Does this work with dma-buf or does dma-buf not allow swiotlb bouncing?

Currently, it does work with dma-buf. OTOH Christoph is apparently not
very happy about it and would rather implement alternative mechanisms to
let dma-buf allocate buffers so that they do not require swiotlb. See
his reply here:

  https://lkml.org/lkml/2023/4/7/38

OTOH if you're asking because of swiotlb use by encrypted VM guests,
the answer might be different.

Cheers
Petr T