On Tue, Nov 05, 2013 at 02:09:53PM -0800, Anthony Liguori wrote: > On Tue, Nov 5, 2013 at 1:16 PM, Konrad Rzeszutek Wilk > <konrad.w...@oracle.com> wrote: > > On Tue, Nov 05, 2013 at 12:53:17PM -0800, Anthony Liguori wrote: > >> Matt Wilson <m...@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.w...@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.
I tried a rwlock a while back. It didn't help much because we actually have a problem with concurrent adds/removes as much as lookups. --msw -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
