Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow

From: Gleb Natapov
Date: Mon Oct 22 2012 - 08:58:50 EST

On Mon, Oct 22, 2012 at 02:55:14PM +0200, Jan Kiszka wrote:
> On 2012-10-22 14:53, Gleb Natapov wrote:
> > On Mon, Oct 22, 2012 at 02:45:37PM +0200, Jan Kiszka wrote:
> >> On 2012-10-22 14:18, Avi Kivity wrote:
> >>> On 10/22/2012 01:45 PM, Jan Kiszka wrote:
> >>>
> >>>>> Indeed. git pull, recheck and call for kvm_flush_coalesced_mmio_buffer()
> >>>>> is gone. So this will break new userspace, not old. By global you mean
> >>>>> shared between devices (or memory regions)?
> >>>>
> >>>> Yes. We only have a single ring per VM, so we cannot flush multi-second
> >>>> VGA access separately from other devices. In theory solvable by
> >>>> introducing per-region rings that can be driven separately.
> >>>
> >>> But in practice unneeded. Real time VMs can disable coalescing and not
> >>> use planar VGA modes.
> >>
> >> A) At least right now, we do not differentiate between the VGA modes and
> >> if flushing is needed. So that device is generally taboo for RT cores of
> >> the VM.
> >> B) We need to disable coalescing in E1000 as well - if we want to use
> >> that model.
> >> C) Gleb seems to propose using coalescing far beyond those two use cases.
> >>
> > Since the userspace change is needed the idea is dead, but if we could
> > implement it I do not see how it can hurt the latency if it would be the
> > only mechanism to use coalesced mmio buffer. Checking that the ring buffer
> > is empty is cheap and if it is not empty it means that kernel just saved
> > you a lot of 8 bytes exists so even after iterating over all the entries there
> > you still saved a lot of time.
> When taking an exit for A, I'm not interesting in flushing stuff for B
> unless I have a dependency. Thus, buffers would have to be per device
> before extending their use.
Buts this is not what will happen (in the absence of other users of
coalesced mmio). What will happen is instead of taking 200 exists for B
you will take 1 exit for B.

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