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

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Message-ID: <508542EF.5050401@redhat.com>
Date:	Mon, 22 Oct 2012 14:58:23 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.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 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.

-- 
error compiling committee.c: too many arguments to function
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