linux-kernel - Re: [PATCH 1/7] KVM: userspace interface

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Message-ID: <4537CD94.2070706@qumranet.com>
Date:	Thu, 19 Oct 2006 21:10:12 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Anthony Liguori <aliguori@...ibm.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	John Stoffel <john@...ffel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] KVM: userspace interface

Anthony Liguori wrote:
>
> ioctls are probably wrong here though.  Ideally, you would want to be 
> able to support an SMP guest.  This means you need to have two virtual 
> processors executing in kernel space.  If you use ioctls, it forces 
> you to have two separate threads in userspace.  This would be hard for 
> something like QEMU which is currently single threaded (and not at all 
> thread safe).
>

Since we're using the Linux scheduler, we need a task per virtual cpu 
anyway, so a thread per vcpu is not a problem.


> If you used a read/write interface, you could poll for any number of 
> processors and handle IO emulation in a single userspace thread (which 
> seems closer to how hardware really works anyway).
>

We can still do that by having the thread write an I/O request to 
hardware service thread, and read back the response.  However that will 
not be too good for scheduling.  For now the smp plan is to slap a 
single lock on the qemu device model, and later fine-grain the locking 
on individual devices as necessary.

Qemu's transition to aio will probably help in reducing the amount of 
work done under lock.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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