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

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Message-ID: <5279085E.7070703@citrix.com>
Date:	Tue, 5 Nov 2013 16:01:50 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	David Vrabel <david.vrabel@...rix.com>, <xen-devel@...ts.xen.org>,
	<linux-kernel@...r.kernel.org>, Matt Wilson <msw@...zon.com>
Subject: Re: [PATCH] grant-table: don't set m2p override if kmap_ops is not
 set

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.

With this patch the m2p is correctly provided by mfn_to_pfn_no_overrides
just by using the machine_to_phys_mapping array, why we need to add an
override?
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