linux-kernel - Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap

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Message-ID: <527916EE.6060104@citrix.com>
Date:	Tue, 5 Nov 2013 17:03:58 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Matt Wilson <msw@...zon.com>, <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops
 is not set

On 05/11/13 16:08, Ian Campbell wrote:
> On Tue, 2013-11-05 at 16:01 +0100, Roger Pau Monné wrote:
>> On 05/11/13 15:56, Konrad Rzeszutek Wilk wrote:
>>> On Tue, Nov 05, 2013 at 03:47:08PM +0100, Roger Pau Monné wrote:
>>>> On 05/11/13 13:36, David Vrabel wrote:
>>>>> On 05/11/13 11:24, Roger Pau Monne wrote:
>>>>>> IMHO there's no reason to set a m2p override if the mapping is done in
>>>>>> kernel space, so only set the m2p override when kmap_ops is set.
>>>>>
>>>>> Can you provide a more detailed reasoning about why this is safe?
>>>>
>>>> To tell the truth, I don't understand why we need to use the m2p
>>>> override for kernel space only mappings, my understanding is that this
>>>> m2p override is needed for user space mappings only (where we actually
>>>> end up doing two mappings, one in kernel space and one in user space).
>>>> For kernel space I don't see why we need to do anything else than
>>>> setting the right p2m translation.
>>>
>>> We needed the m2p when doing DMA operations. As the driver would
>>> want the bus address (so p2m) and then when unmapping the DMA we
>>> only get the bus address  - so we needed to do a m2p lookup.
>>
>> OK, we need a m2p (that we already have in machine_to_phys_mapping),
>> what I don't understand is why we need the m2p override.
> 
> The m2p is a host global table.
> 
> For a foreign page grant mapped into the current domain the m2p will
> give you the foreign (owner) domain's p from the m, not the local one.

Yes, you are completely right, then I have to figure out why blkback
works fine with this patch applied (or at least it seems to work fine).

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