netdev - Re: [Xen-devel] [PATCH net-next v3 1/9] xen-netback: Introduce TX grant map definitions

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Message-ID: <52CEE084.5000409@citrix.com>
Date:	Thu, 9 Jan 2014 17:46:44 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, <jonathan.davies@...rix.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Zoltan Kiss <zoltan.kiss@...rix.com>,
	<xen-devel@...ts.xenproject.org>,
	Roger Pau Monné <roger.pau@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [Xen-devel] [PATCH net-next v3 1/9] xen-netback: Introduce TX
 grant map definitions

On 09/01/14 17:28, Stefano Stabellini wrote:
> On Thu, 9 Jan 2014, David Vrabel wrote:
>> On 09/01/14 15:30, Wei Liu wrote:
>>> On Wed, Jan 08, 2014 at 12:10:10AM +0000, Zoltan Kiss wrote:
>>>> This patch contains the new definitions necessary for grant mapping.
>>>>
>>>> v2:
>>>> - move unmapping to separate thread. The NAPI instance has to be scheduled
>>>>   even from thread context, which can cause huge delays
>>>> - that causes unfortunately bigger struct xenvif
>>>> - store grant handle after checking validity
>>>>
>>>> v3:
>>>> - fix comment in xenvif_tx_dealloc_action()
>>>> - call unmap hypercall directly instead of gnttab_unmap_refs(), which does
>>>>   unnecessary m2p_override. Also remove pages_to_[un]map members
>>>
>>> Is it worthy to have another function call
>>> gnttab_unmap_refs_no_m2p_override in Xen core driver, or just add a
>>> parameter to control wether we need to touch m2p_override? I *think* it
>>> will benefit block driver as well?
>>
>> add_m2p_override and remove_m2p_override calls should be moved into the
>> gntdev device as that should be the only user.
> 
> First of all the gntdev device is common code, while the m2p_override is
> an x86 concept.

m2p_add_override() and m2p_remove_override() are already called from
common code and ARM already provides inline stubs.

The m2p override mechanism is also broken by design (local PFN to
foreign MFN may be many-to-one, but the m2p override only works if local
PFN to foreign MFN is one-to-one). So I want the m2p override to be only
used where it is /currently/ necessary.  I think there should be no new
users of it nor should it be considered a fix for any other use case.

David
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