On 07/19/2011 04:03 PM, Sasha Levin wrote:
On Tue, Jul 19, 2011 at 11:51 PM, Anthony Liguori<anthony@xxxxxxxxxxxxx> wrote:
The e1000 is not performance competitive with virtio-net though so it
certainly is reasonable to assume that noone would notice if we removed
coalesced I/O from the e1000.
The point is, it's so incredibly special cased that having it as part of
such a general purpose API seems wrong. Of the hundreds of devices, we only
have one device that we know for sure really needs it and it could easily be
done independent of the memory API for that device.
Sorry for sidetracking it a bit, but if coalesced mmio doesn't fit
nicely in a good memory model, maybe the problem is with coalesced
mmio and not the memory model itself.
Well, I'd say it's the exception that proves the rule :-) I'm really
just advocating that we should treat it as an exception and not try to
change our rules to take it into account.
It's something that's not really being used at the moment - neither in
qemu nor in kvm tools (we just use dirty log in a guest memory block
after finding out coalesced mmio is still slow).
QEMU does use it and it's quite important. Coalesced MMIO is really
about write caching MMIO exits. It only works with devices that have
registers where writing has no side effects. Moreover, it only really
works well when there are lots and lots of writes to these registers
simultaneously.
Couple that with the fact that the buffer is a fixed size and it's
really not flexible enough to be useful for a wide variety of devices.
But for VGA planar mode writes, it works wonders. It would be terrible
to totally lose it. That said, I'm not at all convinced it's useful for
much other than VGA planar mode.
Regards,
Anthony Liguori
What about just deprecating it and doing either the sockets approach
(whenever it's ready), or doing a brand new virtio-memory device to
avoid the possibility of sockets turning into something like coalesced
mmio.
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