[PATCH 0 of 3] x86/mm: Relieve contention of p2m lock on three hot paths

[Andres Lagar-Cavilla <andres@lagarcavilla.org>]

This series is motivated by recent scalability issues reported by Yhan Z Zang
from Intel.

The general pattern used is to perform a synchronized p2m query to take care of
paging in, PoD allocation or unsharing, and then take a reference to the
underlying page. The reference ensures the page can be safely mapped by the
caller as it won't be paged out. The p2m lock can then be relased and not held
while actual work is performed on the page. This reduces p2m locked critical
sections to a minimum.

The pattern is applied to page table walking and emulation of rep movs. By
checking for the bogus zero ram_gpa value on the general emulation function, we
also reduce contention derived from the p2m lock for that hot path.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>

 xen/arch/x86/mm/hap/guest_walk.c |   6 ++++-
 xen/arch/x86/hvm/emulate.c       |  27 +++++++---------------
 xen/arch/x86/hvm/emulate.c       |  48 +++++++++++++++++++++------------------
 3 files changed, 40 insertions(+), 41 deletions(-)