linux-kernel - Re: [PATCH] swiotlb: Max mapping size takes min align mask into account

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Date:   Wed, 11 May 2022 08:02:12 +0200
From:   "hch@....de" <hch@....de>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Tianyu Lan <ltykernel@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "hch@....de" <hch@....de>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>
Subject: Re: [PATCH] swiotlb: Max mapping size takes min align mask into
 account

On Tue, May 10, 2022 at 06:26:55PM +0000, Michael Kelley (LINUX) wrote:
> > Hmm, this seems a bit pessimistic - the offset can vary per mapping, so
> > it feels to me like it should really be the caller's responsibility to
> > account for it if they're already involved enough to care about both
> > constraints. But I'm not sure how practical that would be.
> 
> Tianyu and I discussed this prior to his submitting the patch.
> Presumably dma_max_mapping_size() exists so that the higher
> level blk-mq code can limit the size of I/O requests to something
> that will "fit" in the swiotlb when bounce buffering is enabled.

Yes, the idea that upper level code doesn't need to care was very
much the idea behind dma_max_mapping_size().

> As you mentioned, how else would a caller handle this situation?

Well, we could look at dma_get_min_align_mask in the caller and do
the calculation there, but I really don't think that is a good idea.

So this patch looks sensible to me.