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

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Message-ID: <508556D6.4010802@siemens.com>
Date:	Mon, 22 Oct 2012 16:23:18 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Gleb Natapov <gleb@...hat.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 2012-10-22 16:00, Gleb Natapov wrote:
> On Mon, Oct 22, 2012 at 03:25:49PM +0200, Jan Kiszka wrote:
>> On 2012-10-22 15:08, Gleb Natapov wrote:
>>> 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?
>>
>> Unless I miss a case now, there is no direct access to the framebuffer
>> possible when we are also doing coalescing. Everything needs to go
>> through userspace.
>>
> Yes, with frame buffer is seems to be the case. One can imagine ROMD
> device that is MMIO on write but still can be accessed for read from
> kernel, but it cannot be coalesced even if coalesced buffer is flushed
> on every exit.

Usually, a ROMD device has a stable content as long as it is fast
read/slow write. Once it switches mode, it is slow read as well.

Jan

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