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

Michael S. Tsirkin
2025-01-10T08:32:48Z

On Fri, Jan 10, 2025 at 11:32:46AM +0800, Jason Wang wrote:
> On Wed, Jan 8, 2025 at 7:35 PM Michael S. Tsirkin <mst@redhat.com> 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@redhat.com>
> >
> >
> > So as I said, I do not get why we are optimizing error paths.
> > The commit log at least needs to be improved to document
> > the motivation.
>
> As replied before. Does the following make more sense?
>
> 1) dma_map_sg() did this
> 2) When the driver tries to submit more buffers than SWIOTLB allows,
> dma_map might fail, in these cases, bouncing is useless.
>
> Thanks

About 2 - it's an error path. Is there a reason to think it's common?
About 1 - ok reasonable. You can say "while we have no data on how
common this is, it is consistent with what dma_map_sg does".

--
MST