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

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Message-ID: <5085423C.2020908@redhat.com>
Date:	Mon, 22 Oct 2012 14:55:24 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Jan Kiszka <jan.kiszka@...mens.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 02:53 PM, Gleb Natapov wrote:
> On Mon, Oct 22, 2012 at 02:45:37PM +0200, Jan Kiszka wrote:
>> On 2012-10-22 14:18, Avi Kivity wrote:
>> > On 10/22/2012 01:45 PM, Jan Kiszka wrote:
>> > 
>> >>> Indeed. git pull, recheck and call for kvm_flush_coalesced_mmio_buffer()
>> >>> is gone. So this will break new userspace, not old. By global you mean
>> >>> shared between devices (or memory regions)?
>> >>
>> >> Yes. We only have a single ring per VM, so we cannot flush multi-second
>> >> VGA access separately from other devices. In theory solvable by
>> >> introducing per-region rings that can be driven separately.
>> > 
>> > But in practice unneeded.  Real time VMs can disable coalescing and not
>> > use planar VGA modes.
>> 
>> A) At least right now, we do not differentiate between the VGA modes and
>> if flushing is needed. So that device is generally taboo for RT cores of
>> the VM.
>> B) We need to disable coalescing in E1000 as well - if we want to use
>> that model.
>> C) Gleb seems to propose using coalescing far beyond those two use cases.
>> 
> 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.

It's time where the guest cannot take interrupts, and time in a high
priority guest thread that is spent processing low guest priority requests.


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