RE: [PATCH V6 5/5] net: netvsc: Add Isolation VM support for netvsc driver

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: Michael Kelley (LINUX) <mikelley@microsoft.com>
> Sent: Thursday, December 9, 2021 2:54 PM
> To: Haiyang Zhang <haiyangz@microsoft.com>; Tianyu Lan <ltykernel@gmail.com>; KY
> Srinivasan <kys@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> wei.liu@kernel.org; Dexuan Cui <decui@microsoft.com>; tglx@linutronix.de; mingo@redhat.com;
> bp@alien8.de; dave.hansen@linux.intel.com; x86@kernel.org; hpa@zytor.com;
> davem@davemloft.net; kuba@kernel.org; jejb@linux.ibm.com; martin.petersen@oracle.com;
> arnd@arndb.de; hch@infradead.org; m.szyprowski@samsung.com; robin.murphy@arm.com; Tianyu
> Lan <Tianyu.Lan@microsoft.com>; thomas.lendacky@amd.com
> Cc: iommu@lists.linux-foundation.org; linux-arch@vger.kernel.org; linux-
> hyperv@vger.kernel.org; linux-kernel@vger.kernel.org; linux-scsi@vger.kernel.org;
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> dave.hansen@intel.com
> Subject: RE: [PATCH V6 5/5] net: netvsc: Add Isolation VM support for netvsc driver
> 
> From: Haiyang Zhang <haiyangz@microsoft.com> Sent: Wednesday, December 8, 2021 12:14 PM
> > > From: Tianyu Lan <ltykernel@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