linux-kernel - Re: [PATCH] xen-blkback: only attach blkback if the required features are met

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Message-ID: <5342DC54.2070608@citrix.com>
Date:	Mon, 7 Apr 2014 18:11:48 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
CC:	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH] xen-blkback: only attach blkback if the required features
 are met

On 04/04/14 16:13, Roger Pau Monné wrote:
> On 04/04/14 17:01, David Vrabel wrote:
>> On 04/04/14 15:41, Roger Pau Monne wrote:
>>> Blkback cannot work properly on auto-translated guests if Xen doesn't
>>> update the IOMMU when performing grant maps/unmaps, so only attach if
>>> the newly introduced XENFEAT_hvm_gntmap_supports_iommu is found.
>>
>> Can you explain the problem in more detail and which guest
>> configurations are affected?
>>
>> This isn't a problem that is specific to blkback, but any backend that
>> grant maps and passes the foreign pages to a device so there needs to be
>> a generic solution.
>>
>> Why can't this be fixed by having the swiotlb bounce foreign pages?
> 
> Yes, it could be fixed with that, FreeBSD has been doing it for a long time.
> 
> But since nobody has complained about this, and the issue has been there
> for a long time, I'm not sure it's worth adding all the bounce buffer
> code for what seems to be an unused configuration.

Ok.  Now that netback also grant maps.  Can you make this a generic
function and call from netback and blkback?

David
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