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

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Date:   Thu, 9 Dec 2021 19:54:25 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Haiyang Zhang <haiyangz@...rosoft.com>,
        Tianyu Lan <ltykernel@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
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        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
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        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
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        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>
Subject: RE: [PATCH V6 5/5] net: netvsc: Add Isolation VM support for netvsc
 driver

From: Haiyang Zhang <haiyangz@...rosoft.com> Sent: Wednesday, December 8, 2021 12:14 PM
> > From: Tianyu Lan <ltykernel@...il.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?

Michael