Re: [Qemu-devel] Avoiding nographic_timer exits

[Jan Kiszka]

On 2011-09-30 10:18, David Gibson wrote:
> With PowerKVM, exits from KVM to qemu are even more expensive than on
> x86.  One significant source of these we're finding (since we usually
> work in -nographic mode) is the nographic_timer.
> 
> At present, we're using a hack to disable it, but that's obviously not
> a long term solution.  From examination, it looks like the only
> purpose of this timer is to flush coalesced mmios.  So it seems like
> the timer should only be activated when a coalesced mmio region
> actually exists, but I'm not entirely sure how to go about that.
> 
> Thinking longer term, it seems very odd that a userspace periodic
> timer handles this at all.  Surely it would make more sense to use a
> kernel timer within KVM, which can be activated only when there are
> actually pending coalesced MMIOs in the buffer.

Coalesced MMIO should only be flushed when a device depending on it gets
accessed - either by a VCPU or by the iothread (to update the graphic
output e.g.). We are working on such a concept (to reduce latency for
VCPUs with real-time constraints).

And long-term, we need per-device MMIO buffers to avoid flushing
unrelated content.

Jan

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