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

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Message-ID: <50853FF1.8010809@siemens.com>
Date:	Mon, 22 Oct 2012 14:45:37 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.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 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.

Jan

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