Messages in this thread |  | | Date | Tue, 5 Nov 2013 14:09:53 -0800 | Subject | Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set | From | Anthony Liguori <> |
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On Tue, Nov 5, 2013 at 1:16 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote: > On Tue, Nov 05, 2013 at 12:53:17PM -0800, Anthony Liguori wrote: >> 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. > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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