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

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Date:	Fri, 4 Apr 2014 17:13:54 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	David Vrabel <david.vrabel@...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 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.

Also, since Linux also supports PV, I would just say that if you want to
use driver domains you are forced to use PV guests (or have a recent Xen
version that properly updates the IOMMU on grant map/unmap).

>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -1381,6 +1381,18 @@ static int __init xen_blkif_init(void)
>>  	if (!xen_domain())
>>  		return -ENODEV;
>>  
>> +#ifdef CONFIG_X86
>> +	if (xen_feature(XENFEAT_auto_translated_physmap) &&
>> +	    !xen_feature(XENFEAT_hvm_gntmap_supports_iommu)) {
>> +		/*
>> +		 * blkback cannot work properly on auto-translated guests
>> +		 * if grant table mappings don't update the IOMMU entries.
>> +		 */
>> +		pr_debug(DRV_PFX "Disabling blkback because Xen is missing feature XENFEAT_hvm_gntmap_supports_iommu\n");
>> +		return -ENODEV;
>> +	}
>> +#endif /* CONFIG_X86 */
> 
> Why CONFIG_X86?

ARM has a bounce buffer for foreign pages, so this is not needed.

> 
> David
> 

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