Messages in this thread |  | | Date | Wed, 13 Nov 2013 10:36:54 +0000 | From | David Vrabel <> | Subject | Re: [PATCH] xen-gnttab: do not add m2p override entries for blkback mappings |
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On 13/11/13 01:48, Anthony Liguori wrote: > From: Anthony Liguori <aliguori@amazon.com> > > Commit 5dc03639 switched blkback to also add m2p override entries > when mapping grant pages but history seems to have forgotten why > this is useful if it ever was. > > The blkback driver does not need m2p override entries to exist > and there is significant overhead due to the locking in the m2p > override table. We see about a 10% improvement in IOP rate with > this patch applied running FIO in the guest. > > See http://article.gmane.org/gmane.linux.kernel/1590932 for a full > analysis of current users.
I think it would be better if it was made clearer what m2p_add/remove_override() is for (i.e., allowing get_user_pages_fast() to work) so there isn't this confusion in the future. Please add some comments for this and move the calls into the gntdev driver.
In the future I would like to see the grant map/unmap done in m2p_add/remove_override() moved into gntdev as well, but this isn't a requirement for this performance fix.
As a prerequiste, the call to dma_mark_clean() in xen_swiotlb_unmap() needs to be removed and replaced with a comment explaining why this now differs from the generic implementation. i.e., the necessary phys_to_virt() will not work for foreign pages and dma_mark_clean() is a nop on all Xen supported architectures.
David
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