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

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Message-ID: <5085428A.9050401@redhat.com>
Date:	Mon, 22 Oct 2012 14:56:42 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.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 10/22/2012 02:45 PM, 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.

In non-planar modes the memory will be direct mapped, which overrides
coalescing (since kvm or qemu never see an exit).

> B) We need to disable coalescing in E1000 as well - if we want to use
> that model.

True.

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