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

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Date:	Mon, 22 Oct 2012 15:01:02 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...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 Mon, Oct 22, 2012 at 02:55:24PM +0200, Avi Kivity wrote:
> 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.
> 
Proposed fix has exactly same issue. Until all data is transfered to
userspace no interrupt will be served.

--
			Gleb.
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