linux-kernel - Re: [PATCH] dma: DMA_ATTR_SKIP_CPU

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Date:   Thu, 20 Jul 2023 02:21:05 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, linux-doc@...r.kernel.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks

On Thu, Jul 20, 2023 at 08:07:42AM +0200, Christoph Hellwig wrote:
> On Wed, Jul 19, 2023 at 06:15:59AM -0400, Michael S. Tsirkin wrote:
> > A recent patchset highlighted to me that DMA_ATTR_SKIP_CPU_SYNC
> > might be easily misunderstood.
> 
> .. just curious: what patchset is that?  DMA_ATTR_SKIP_CPU_SYNC is
> often a bad idea and all users probably could use a really good
> audit..

Message-Id: <20230710034237.12391-1-xuanzhuo@...ux.alibaba.com>


Looks like there's really little else can be done: there's a
shared page we allow DMA into, so we sync periodically.
Then when we unmap we really do not need that data
synced again.

What exactly is wrong with this?


> >  #define DMA_ATTR_NO_KERNEL_MAPPING	(1UL << 4)
> >  /*
> > - * DMA_ATTR_SKIP_CPU_SYNC: Allows platform code to skip synchronization of
> > - * the CPU cache for the given buffer assuming that it has been already
> > - * transferred to 'device' domain.
> > + * DMA_ATTR_SKIP_CPU_SYNC: Allows platform code to skip synchronization of the
> > + * CPU and device domains for the given buffer.
> 
> While we're at it, I think "allows" is the wrong word here, we really
> must skip the synchronization or else we're in trouble.

Hmm could you explain? I thought multiple sync operations are harmless.

-- 
MST