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

From: Jan Kiszka
Date: Mon Oct 22 2012 - 09:06:04 EST

On 2012-10-22 14:58, Avi Kivity wrote:
> On 10/22/2012 02:55 PM, Jan Kiszka wrote:
>>> 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.
> Any mmio exit has to flush everything. For example a DMA caused by an
> e1000 write has to see any writes to the framebuffer, in case the guest
> is transmitting its framebuffer to the outside world.

We already flush when that crazy guest actually accesses the region, no
need to do this unconditionally.


Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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