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

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Message-ID: <20131105200616.GA23362@u109add4315675089e695.ant.amazon.com>
Date:	Tue, 5 Nov 2013 12:06:16 -0800
From:	Matt Wilson <msw@...ux.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	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,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if
 kmap_ops is not set

On Tue, Nov 05, 2013 at 05:03:58PM +0100, Roger Pau Monné wrote:
> 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).

blkback also works for me when testing a similar patch. I'm still
confused. One thing with your proposed patch: I'm not sure that you're
putting back the correct mfn.

Adding Anthony to the thread.

--msw

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