[RFC PATCH 0/3] Add an option for devices to skip SWIOTLB pre-copy on read.

[Keith Busch]

On Wed, Feb 05, 2025 at 04:43:21PM +0100, Christoph Hellwig wrote:
> On Wed, Feb 05, 2025 at 09:54:21AM +0200, Andy Shevchenko wrote:
> > On Tue, Feb 04, 2025 at 11:36:27PM +0000, Jianxiong Gao wrote:
> > > Removes an extra memory copy that occurs during IO read
> > > operations through the SWIOTLB. During high throughput
> > > read workloads, this extra copy is causing a lot of stress
> > > on the SWIOTLB.
> > > 
> > > With high performance devices, for example NVMe devices,
> > > the device will be overwriting the entire buffer.
> > 
> > Is this really guaranteed? I can imagine surprise power cut or
> > hotplug event, for example, just in the middle of the transfer.
> 
> Many command can return less data than originally mapped.  Get Log Page
> is an example that comes to mind that is heavily used that way.

It's also easy enough for a user to "mistakenly" request more memory
than the device will transfer. The safest thing is to clear any contents
that may get copied back to the user.