Re: [PATCH 7/7] Allow KVM from a normal QEMU binary

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Anthony Liguori wrote:
> Avi Kivity wrote:
>> Anthony Liguori wrote:
>>> One of the things I was thinking of doing was getting rid of USE_KVM 
>>> completely.  What I was thinking of doing is moving kvmctl.[ch] into 
>>> the QEMU tree
>>
>> I'd like to keep kvmctl out, so that non-qemu based userspace can be 
>> built.  True, I wouldn't like to be the one to do it, but I'd like to 
>> keep the possibility open.
>
> Yeah, I'm on the fence myself.  In the very least, we need a smarter 
> configure check in QEMU to detect the presence of libkvm and enable 
> the define appropriately.
>
> I'll go as far as to say that once there's proper configure checks 
> (and kvm fails gracefully), it would be appropriate to send it to 
> qemu-devel.  What are your thoughts?

Qemu-devel has been strangely nonresponsive to my patches (the x86-64 
gdb patch was sent at least two months ago).  Maybe as a known 
contributor they'll be more responsive to you.

The only other issue is how stable the interface is.  I think that the 
kvmctl interface is almost there (I'd like to add a libkvm_open() and 
libkvm_close(), have kvm_create() use the context returned from 
libkvm_open(), and not create a vcpu and memory in kvm_create(), but 
these are fairly cosmetic).

The kernel interface is another matter.  I want to apply Arnd's 
suggestions, but that's some time off.  If we don't push libkvm/kvmctl 
to qemu then that's not an issue.

>>
>> State transfer is what turned us of qemu.  Reconstructing the state 
>> (esp. eflags and the segments) is quite difficult and there are some 
>> annoying qemu bugs (like the busy task flag checks).
>
> The Xen tree already has the conversion code.  It's annoying but it's 
> not nearly as bad as say, shadow paging :-)

Well, a nice property of real mode is that it doesn't use shadow paging :)

>
> While not necessarily in your scope, in the long term, having the 
> state transfer would be really useful.
>
> I can imagine KVM eventually supporting VT, SVM, user mode 
> virtualization, and paravirtualization. State transfer use is useful 
> for VT at least but is absolutely required for user mode virtualization.  

What is user mode virtualization in this context?

> All of these things exist in some form right now (user mode via qvm86 
> and para via lhype) and all are duplicating a lot of tough code (such 
> as shadow paging).

Please elaborate.

>
> Of course, state transfer won't be impossible to add down the road at 
> some point so if you think big real mode will be easier with 
> emulation, it wouldn't be that bad to give it a shot.
>

We already had real mode (and 16-bit protected, and 32-bit protected) 
during development.  It should be very easy to add it back.  I'm worried 
about maintenance and things like smp.


>> BTW, dumping qemu cpu emulation will allow us to use gcc 4.x instead 
>> of gcc 3.x.
>>
>>>
>>> SMP guest support is still a hard problem that needs thinking 
>>> through but if Xen solves it, KVM can use the solution (or vice versa).
>>
>> I don't see a huge problem.  We'll have to get the APIC right, and 
>> the mmu locking right in the kernel, but what else?
>
> It's not so bad provided you don't use QEMU for emulation.  That's 
> what I was referring to.  There are some tricky cases where QEMU is 
> accessing a piece of memory with an atomic instruction while another 
> VCPU is running on bare metal and trying to touch the same memory.  I 
> suspect that as-is, QEMU would violate the atomic guarantees when it 
> translates the instruction.

Ok.  We're on the same cacheline then.  An emulator definitely solves this.

(though I can't see real-mode locked instructions happening... maybe 
that's an issue with qemu emulating multiple instructions for Xen during 
mmio)


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


-------------------------------------------------------------------------
Using Tomcat but need to do more? Need to support web services, security?
Get stuff done quickly with pre-integrated technology to make your job easier
Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo
http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642