linux-kernel - Re: [PATCH v1 4/4] swiotlb: panic if nslabs is too small

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Message-ID: <YwNn92WP3rP4ylZu@infradead.org>
Date:   Mon, 22 Aug 2022 04:26:47 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Yu Zhao <yuzhao@...gle.com>, dongli.zhang@...cle.com,
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Subject: Re: [PATCH v1 4/4] swiotlb: panic if nslabs is too small

On Mon, Aug 22, 2022 at 10:49:09AM +0100, Robin Murphy wrote:
> Hmm, it's possible this might be quietly fixed by 20347fca71a3, but either
> way I'm not sure why we would need to panic *before* we've even tried to
> allocate anything, when we could simply return with no harm done? If we've
> ended up calculating (or being told) a buffer size which is too small to be
> usable, that should be no different to disabling SWIOTLB entirely.

Hmm.  I think this might be a philosophical question, but I think
failing the boot with a clear error report for a configuration that is
supposed to work but can't is way better than just panicing later on.

> Historically, passing "swiotlb=1" on the command line has been used to save
> memory when the user knows SWIOTLB isn't needed. That should definitely not
> be allowed to start panicking.

I've never seen swiotlb=1 advertized as a way to disable swiotlb.
That's always been swiotlb=noforce, which cleanly disables it.