Re: [PATCH] virtio_ring: skip cpu sync when mapping fails

[Michael S. Tsirkin]

On Mon, Nov 11, 2024 at 04:36:52PM +0800, Jason Wang wrote:
> On Mon, Nov 11, 2024 at 3:30 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> > On Mon, Nov 11, 2024 at 10:55:38AM +0800, Jason Wang wrote:
> > > There's no need to sync DMA for CPU on mapping errors. So this patch
> > > skips the CPU sync in the error handling path of DMA mapping.
> > >
> > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> >
> > DMA sync is idempotent.
> > Extra work for slow path.  Why do we bother?
> 
> dma_map_sg() did this, since current virtio hack sg mappings to per
> page mapping, we lose such optimization.
> 
> Actually the path is not necessarily slowpath in some setups like
> swiotlb or VDUSE.
> 
> Thanks

I don't get how it's not a slowpath. Example?

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MST