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SubjectRe: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set
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Matt Wilson <msw@linux.com> writes:

> 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.

It's perfectly fine to store a foreign pfn in the m2p table. The m2p
override table is used by the grant device to allow a reverse lookup of
the real mfn to a pfn even if it's foreign.

blkback doesn't actually need this though. This was introduced in:

commit 5dc03639cc903f887931831d69895facb5260f4b
Author: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Tue Mar 1 16:46:45 2011 -0500

xen/blkback: Utilize the M2P override mechanism for GNTMAP_host_map

Purely as an optimization. In practice though due to lock contention it
slows things down.

I think an alternative would be to use a read/write lock instead of just
a spinlock since it's the read path that is the most hot.

I haven't tested that yet though.

Regards,

Anthony Liguori

>
> Adding Anthony to the thread.
>
> --msw
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