Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

On Tue, Nov 5, 2013 at 1:16 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Nov 05, 2013 at 12:53:17PM -0800, Anthony Liguori wrote:
>> Matt Wilson <msw@xxxxxxxxx> 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@xxxxxxxxxx>
>> 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.
>
> The m2p hash table can also be expanded to lower the contention.

That was the first thing I tried :-)

The issue isn't lookup cost as much as the number of acquisitions and
the cost of adding/removing entries.

>> I haven't tested that yet though.
>
> Looking forward to your patches :-)

I'm really just being thorough in suggesting the rwlock.  I actually
think that not using the m2p override table is the right long term
fix.

Regards,

Anthony Liguori

>>
>> Regards,
>>
>> Anthony Liguori
>>
>> >
>> > Adding Anthony to the thread.
>> >
>> > --msw

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