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Date: Wed, 28 Oct 2015 13:30:59 +0100
From: Igor Mammedov <imammedo@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 0/6] virtio: handle non contigious s/g
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com

On Tue, 27 Oct 2015 13:51:20 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Oct 27, 2015 at 10:47:54AM +0200, Michael S. Tsirkin wrote:
> > TL;DR:
> > This fixes virtio in a way transparent to guest.
> > We should now be able to revert commits aa8580cd and df0acded19ec which worked
> > around it in a way that's not transparent.
> 
> I didn't check dataplane BTW. Igor? Stefan?
verified that series fixes virtio-[blk|scsi|net], all of them
hit at least one descriptor(indirect) that crosses DIMM boundary
and QEMU survived it.

However as Stefan has said virtio-blk with dataplane enabled hangs
guest instead of QEMU crashing and QEMU prints following error:

"Failed to map descriptor addr 0x1045eb000 len 106496"

I've used following CLI:
qemu-system-x86_64 -enable-kvm -enable-kvm  -m 128M,slots=250,maxmem=32G  -drive if=none,id=hd,file=rhel72.img,cache=none,aio=native,format=raw -device virtio-blk,drive=hd,scsi=off,config-wce=off,x-data-plane=on `for i in $(seq 0 15); do echo -n "-object memory-backend-ram,id=m$i,size=10M -device pc-dimm,id=dimm$i,memdev=m$i "; done`

it hangs at boot time or on executing 'dd if=/dev/vda of=/dev/null bs 32M'