linux-kernel - Re: [PATCH] KVM: x86: fix vcpu->mmio

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Date:	Mon, 22 Oct 2012 15:05:58 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow

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.

Jan

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