[Qemu-devel] [PATCH v3 uq/master 0/6] kvm: Get coalesced MMIO flushing out of the hot-path

[Jan Kiszka]

This is just a repost, now targeting uq/master as agreed. No changes
compared to v2 except that "i82378: Remove bogus MMIO coalescing" was
dropped as it is already in QEMU upstream by now.

Original description:

We currently flush the coalesced MMIO buffer on every vmexit to
userspace. KVM only provides a single buffer per VM, so a central lock
is required to read from it. This is a contention point given a large
enough VCPU set. Moreover, we need to hold the BQL while replaying the
queued requests, probably for a long time until there is more fine
grained locking available. Good reasons to overcome the unconditional
flush.

The series achieves this by flushing only on selected memory region
accesses, either generically via the memory access dispatcher or
directly on certain VGA PIO accesses that are not yet fully converted.
Another reason to flush are remappings or other relevant region state
changes.

Jan Kiszka (6):
  memory: Flush coalesced MMIO on selected region access
  memory: Use transaction_begin/commit also for single-step operations
  memory: Fold memory_region_update_topology into
    memory_region_transaction_commit
  memory: Flush coalesced MMIO on mapping and state changes
  VGA: Flush coalesced MMIO on related MMIO/PIO accesses
  kvm: Stop flushing coalesced MMIO on vmexit

 hw/cirrus_vga.c |    7 ++++
 hw/qxl.c        |    1 +
 hw/vga-isa-mm.c |    1 +
 hw/vga.c        |    5 +++
 hw/vmware_vga.c |    1 +
 kvm-all.c       |    2 -
 memory.c        |  104 ++++++++++++++++++++++++++++++++-----------------------
 memory.h        |   26 ++++++++++++++
 8 files changed, 102 insertions(+), 45 deletions(-)

-- 
1.7.3.4