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

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Date:	Thu, 9 Jan 2014 17:28:21 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	David Vrabel <david.vrabel@...rix.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 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.

Then I would like to point out that there are no guarantees that a
network driver, or any other kernel subsystems, don't come to rely on
mfn_to_pfn translations for any reasons at any time.
It just happens that today the only known user is gupf, but tomorrow,
who knows?
If we move the m2p_override calls to the gntdev device somehow (avoif
ifdefs please), we should be very well aware of the risks involved.

Of course my practical self realizes that we don't want a performance
regression and this is the quickest way to fix it, so I am not
completely oppose to it.
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