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Message-ID: <972d7b71-d126-0a00-2772-030c12f35e0e@huaweicloud.com>
Date:   Wed, 22 Mar 2023 15:32:30 +0100
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Subject: Re: [PATCH 1/1] swiotlb: Track and report io_tlb_used high water mark
 in debugfs
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To:     Michael Kelley <mikelley@microsoft.com>, hch@lst.de,
        m.szyprowski@samsung.com, robin.murphy@arm.com,
        decui@microsoft.com, tiala@microsoft.com, iommu@lists.linux.dev,
        linux-kernel@vger.kernel.org
References: <1677516106-85971-1-git-send-email-mikelley@microsoft.com>
From:   Petr Tesarik <petrtesarik@huaweicloud.com>
In-Reply-To: <1677516106-85971-1-git-send-email-mikelley@microsoft.com>
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Hi,

On 2/27/2023 5:41 PM, Michael Kelley wrote:
> swiotlb currently reports the total number of slabs and the instantaneous
> in-use slabs in debugfs. But with increased usage of swiotlb for all I/O
> in Confidential Computing (coco) VMs, it has become difficult to know
> how much memory to allocate for swiotlb bounce buffers, either via the
> automatic algorithm in the kernel or by specifying a value on the
> kernel boot line. The current automatic algorithm generously allocates
> swiotlb bounce buffer memory, and may be wasting significant memory in
> many use cases.

I like the idea. Tracking and exposing the high watermark is definitely a step in the right direction. I would also appreciate an indicator of fragmentation, but that can be implemented later.

However, you apparently have a scenario where swiotlb is used intensely. Are you able to measure swiotlb performance? If yes, can you suggest a suitable method? I'm asking for my work on making swiotlb more flexible (able to grow and shrink).

Petr T