Re: [PATCH] xen-gnttab: do not add m2p override entries for blkback mappings

[David Vrabel <david.vrabel@citrix.com>]

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