linux-kernel - Re: [PATCH v2 0/7] Allow dynamic allocation of software IO TLB bounce buffers

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Date:   Wed, 26 Apr 2023 14:44:39 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Petr Tesarik <petrtesarik@...weicloud.com>,
        Jonathan Corbet <corbet@....net>,
        "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>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Borislav Petkov <bp@...e.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Kim Phillips <kim.phillips@....com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Ondrej Zary <linux@...y.sk>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Petr Tesarik <petr.tesarik.ext@...wei.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Won Chung <wonchung@...gle.com>,
        "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 0/7] Allow dynamic allocation of software IO TLB
 bounce buffers

Hi Greg,

On Wed, 26 Apr 2023 14:26:36 +0200
Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:

> On Wed, Apr 26, 2023 at 02:15:20PM +0200, Petr Tesařík wrote:
> > Hi,
> > 
> > On Wed, 19 Apr 2023 12:03:52 +0200
> > Petr Tesarik <petrtesarik@...weicloud.com> wrote:
> >   
> > > From: Petr Tesarik <petr.tesarik.ext@...wei.com>
> > > 
> > > The goal of my work is to provide more flexibility in the sizing of
> > > SWIOTLB.
> > > 
> > > The software IO TLB was designed with these assumptions:
> > > 
> > > 1. It would not be used much, especially on 64-bit systems.
> > > 2. A small fixed memory area (64 MiB by default) is sufficient to
> > >    handle the few cases which require a bounce buffer.
> > > 3. 64 MiB is little enough that it has no impact on the rest of the
> > >    system.
> > > 
> > > First, if SEV is active, all DMA must be done through shared
> > > unencrypted pages, and SWIOTLB is used to make this happen without
> > > changing device drivers. The software IO TLB size is increased to
> > > 6% of total memory in sev_setup_arch(), but that is more of an
> > > approximation. The actual requirements may vary depending on the
> > > amount of I/O and which drivers are used. These factors may not be
> > > know at boot time, i.e. when SWIOTLB is allocated.
> > > 
> > > Second, other colleagues have noticed that they can reliably get
> > > rid of occasional OOM kills on an Arm embedded device by reducing
> > > the SWIOTLB size. This can be achieved with a kernel parameter, but
> > > determining the right value puts additional burden on pre-release
> > > testing, which could be avoided if SWIOTLB is allocated small and
> > > grows only when necessary.  
> > 
> > Now that merging into 6.4 has begun, what about this patch series? I'm
> > eager to get some feedback (positive or negative) and respin the next
> > version.  
> 
> It's the merge window, we can't add new things that haven't been in
> linux-next already.

This is understood. I'm not asking for immediate inclusion.

>   Please resubmit it after -rc1 is out.

If you can believe that rebasing to -rc1 will be enough, then I will
also try to believe I'm lucky. ;-)

The kind of feedback I really want to get is e.g. about the extra
per-device DMA-specific fields. If they cannot be added to struct
device, then I'd rather start discussing an interim solution, because
getting all existing DMA fields out of that struct will take a lot of
time...

Thanks,
Petr T