linux-kernel - Re: [PATCH] 9p/trans_virtio.c: Fix broken zero-copy on vmalloc() buffers

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Date:	Thu, 30 Jan 2014 16:29:26 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ryao@...too.org
Cc:	ericvh@...il.com, rminnich@...dia.gov, lucho@...kov.net,
	v9fs-developer@...ts.sourceforge.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel@...too.org,
	aneesh.kumar@...ux.vnet.ibm.com, will.deacon@....com,
	cov@...eaurora.org, behlendorf1@...l.gov, mthode@...ode.org
Subject: Re: [PATCH] 9p/trans_virtio.c: Fix broken zero-copy on vmalloc()
 buffers

From: Richard Yao <ryao@...too.org>
Date: Thu, 30 Jan 2014 13:02:48 -0500

> The 9p-virtio transport does zero copy on things larger than 1024 bytes
> in size. It accomplishes this by returning the physical addresses of
> pages to the virtio-pci device. At present, the translation is usually a
> bit shift.
> 
> However, that approach produces an invalid page address when we
> read/write to vmalloc buffers, such as those used for Linux kernle
> modules. This causes QEMU to die printing:
> 
> qemu-system-x86_64: virtio: trying to map MMIO memory
> 
> This patch enables 9p-virtio to correctly handle this case. This not
> only enables us to load Linux kernel modules off virtfs, but also
> enables ZFS file-based vdevs on virtfs to be used without killing QEMU.
> 
> Also, special thanks to both Avi Kivity and Alexander Graf for their
> interpretation of QEMU backtraces. Without their guidence, tracking down
> this bug would have taken much longer.
> 
> Signed-off-by: Richard Yao <ryao@...too.org>
> Acked-by: Alexander Graf <agraf@...e.de>
> Reviewed-by: Will Deacon <will.deacon@....com>

Applied, thanks.
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