> -----Original Message-----
> From: Michael Kelley (LINUX) <mikel...@microsoft.com>
> Sent: Thursday, December 9, 2021 2:54 PM
> To: Haiyang Zhang <haiya...@microsoft.com>; Tianyu Lan <ltyker...@gmail.com>;
> KY
> Srinivasan <k...@microsoft.com>; Stephen Hemminger <sthem...@microsoft.com>;
> wei....@kernel.org; Dexuan Cui <de...@microsoft.com>; t...@linutronix.de;
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> robin.mur...@arm.com; Tianyu
> Lan <tianyu....@microsoft.com>; thomas.lenda...@amd.com
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> Subject: RE: [PATCH V6 5/5] net: netvsc: Add Isolation VM support for netvsc
> driver
>
> From: Haiyang Zhang <haiya...@microsoft.com> Sent: Wednesday, December 8,
> 2021 12:14 PM
> > > From: Tianyu Lan <ltyker...@gmail.com>
> > > Sent: Tuesday, December 7, 2021 2:56 AM
>
> [snip]
>
> > > static inline int netvsc_send_pkt(
> > > struct hv_device *device,
> > > struct hv_netvsc_packet *packet,
> > > @@ -986,14 +1105,24 @@ static inline int netvsc_send_pkt(
> > >
> > > trace_nvsp_send_pkt(ndev, out_channel, rpkt);
> > >
> > > + packet->dma_range = NULL;
> > > if (packet->page_buf_cnt) {
> > > if (packet->cp_partial)
> > > pb += packet->rmsg_pgcnt;
> > >
> > > + ret = netvsc_dma_map(ndev_ctx->device_ctx, packet, pb);
> > > + if (ret) {
> > > + ret = -EAGAIN;
> > > + goto exit;
> > > + }
> >
> > Returning EAGAIN will let the upper network layer busy retry,
> > which may make things worse.
> > I suggest to return ENOSPC here like another place in this
> > function, which will just drop the packet, and let the network
> > protocol/app layer decide how to recover.
> >
> > Thanks,
> > - Haiyang
>
> I made the original suggestion to return -EAGAIN here. A
> DMA mapping failure should occur only if swiotlb bounce
> buffer space is unavailable, which is a transient condition.
> The existing code already stops the queue and returns
> -EAGAIN when the ring buffer is full, which is also a transient
> condition. My sense is that the two conditions should be
> handled the same way. Or is there a reason why a ring
> buffer full condition should stop the queue and retry, while
> a mapping failure should drop the packet?
The netvsc_dma_map() fails in these two places. The dma_map_single()
is doing the maping with swiotlb bounce buffer, correct? And it will
become successful after the previous packets are unmapped?
+ packet->dma_range = kcalloc(page_count,
+ sizeof(*packet->dma_range),
+ GFP_KERNEL);
+ dma = dma_map_single(&hv_dev->device, src, len,
+ DMA_TO_DEVICE);
I recalled your previous suggestion now, and agree with you that
we can treat it the same way (return -EAGAIN) in this case. And
the existing code will stop the queue temporarily.
Thanks,
- Haiyang
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