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Date:   Wed, 26 Apr 2023 14:15:20 +0200
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Petr Tesarik <petrtesarik@huaweicloud.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
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        Robin Murphy <robin.murphy@arm.com>,
        Borislav Petkov <bp@suse.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
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        Muchun Song <muchun.song@linux.dev>,
        Ondrej Zary <linux@zary.sk>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Petr Tesarik <petr.tesarik.ext@huawei.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
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        Roberto Sassu <roberto.sassu@huawei.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
Subject: Re: [PATCH v2 0/7] Allow dynamic allocation of software IO TLB
 bounce buffers
Message-ID: <20230426141520.0caf4386@meshulam.tesarici.cz>
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Hi,

On Wed, 19 Apr 2023 12:03:52 +0200
Petr Tesarik <petrtesarik@huaweicloud.com> wrote:

> From: Petr Tesarik <petr.tesarik.ext@huawei.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.

Petr T