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

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Message-ID: <20121022130810.GW29310@redhat.com>
Date:	Mon, 22 Oct 2012 15:08:10 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Avi Kivity <avi@...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 Mon, Oct 22, 2012 at 03:05:58PM +0200, Jan Kiszka wrote:
> 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.
> 
What if framebuffer is accessed from inside the kernel? Is this case handled?

--
			Gleb.
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