linux-kernel - Re: [Qemu-devel] [RFC] Next gen kvm api

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Date:	Wed, 15 Feb 2012 15:59:33 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Avi Kivity <avi@...hat.com>
CC:	Rusty Russell <rusty@...abs.org>,
	qemu-devel <qemu-devel@...gnu.org>,
	KVM list <kvm@...r.kernel.org>, Gleb Natapov <gleb@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

On 02/15/2012 07:39 AM, Avi Kivity wrote:
> On 02/07/2012 08:12 PM, Rusty Russell wrote:
>>> I would really love to have this, but the problem is that we'd need a
>>> general purpose bytecode VM with binding to some kernel APIs.  The
>>> bytecode VM, if made general enough to host more complicated devices,
>>> would likely be much larger than the actual code we have in the kernel now.
>>
>> We have the ability to upload bytecode into the kernel already.  It's in
>> a great bytecode interpreted by the CPU itself.
>
> Unfortunately it's inflexible (has to come with the kernel) and open to
> security vulnerabilities.

I wonder if there's any reasonable way to run device emulation within the 
context of the guest.  Could we effectively do something like SMM?

For a given set of traps, reflect back into the guest quickly changing the 
visibility of the VGA region. It may require installing a new CR3 but maybe that 
wouldn't be so bad with VPIDs.

Then you could implement the PIT as guest firmware using kvmclock as the time base.

Once you're back in the guest, you could install the old CR3.  Perhaps just hide 
a portion of the physical address space with the e820.

Regards,

Anthony Liguori

>> If every user were emulating different machines, LPF this would make
>> sense.  Are they?
>
> They aren't.
>
>> Or should we write those helpers once, in C, and
>> provide that for them.
>
> There are many of them: PIT/PIC/IOAPIC/MSIX tables/HPET/kvmclock/Hyper-V
> stuff/vhost-net/DMA remapping/IO remapping (just for x86), and some of
> them are quite complicated.  However implementing them in bytecode
> amounts to exposing a stable kernel ABI, since they use such a vast
> range of kernel services.
>

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