Re: [PATCH] Add MCE support to KVM

[Gerd Hoffmann]

On 04/20/09 14:43, Avi Kivity wrote:
> Gerd Hoffmann wrote:
> > > That said, I'd like to be able to emulate the Xen HVM hypercalls. But in
> > > any case, they hypercall implementation has to be in the kernel,
> >
> > No. With Xenner the xen hypercall emulation code lives in guest
> > address space.
>
> In this case the guest ring-0 code should trap the #GP, and install the
> hypercall page (which uses sysenter/syscall?). No kvm or qemu changes
> needed.

Doesn't fly.

Reason #1: In the pv-on-hvm case the guest runs on ring0.
Reason #2: Chicken-egg issue:  For the pv-on-hvm case only few,
           simple hypercalls are needed.  The code to handle them
           is small enougth that it can be loaded directly into the
           hypercall page(s).

pure-pv doesn't need it in the first place.  But, yes, there I could 
simply trap #GP because the guest kernel runs on ring #1 (or #3 on 64bit).

> > > Especially if we need to support
> > > tricky bits like continuations.
> >
> > Is there any reason to? I *think* xen does it for better scheduling
> > latency. But with xen emulation sitting in guest address space we can
> > schedule the guest at will anyway.
>
> It also improves latency within the guest itself. At least I think that
> what was the Hyper-V spec is saying. You can interrupt the execution of
> a long hypercall, inject and interrupt, and resume. Sort of like a
> rep/movs instruction, which the cpu can and will interrupt.

Hmm.  Needs investigation..  I'd expect the main source of latencies is 
page table walking.  Xen works very different from kvm+xenner here ...

> For Xenner, no (and you don't need to intercept the msr at all),  but for
> pv-on-hvm, you do need to update the code.

Xenner handling pv-on-hvm doesn't need code updates either.  Real Xen 
does as it uses vmcall, not sure how they handle migration.

cheers
  Gerd
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