Re: [Qemu-devel] [PATCH 0/2 v3] kvm: notify host when guest panicked

[Gleb Natapov]

On Wed, Mar 14, 2012 at 11:46:08AM -0700, Eric Northup wrote:
> On Wed, Mar 14, 2012 at 6:25 AM, Gleb Natapov <address@hidden> wrote:
> 
> > On Wed, Mar 14, 2012 at 03:16:05PM +0200, Avi Kivity wrote:
> > > On 03/14/2012 03:14 PM, Gleb Natapov wrote:
> > > > On Wed, Mar 14, 2012 at 03:07:46PM +0200, Avi Kivity wrote:
> > > > > On 03/14/2012 01:11 PM, Wen Congyang wrote:
> > > > > > >
> > > > > > > I don't think we want to use the driver.  Instead, have a small
> > piece of
> > > > > > > code that resets the device and pushes out a string (the panic
> > message?)
> > > > > > > without any interrupts etc.
> > > > > > >
> > > > > > > It's still going to be less reliable than a hypercall, I agree.
> > > > > >
> > > > > > Do you still want to use complicated and less reliable way?
> > > > >
> > > > > Are you willing to try it out and see how complicated it really is?
> > > > >
> > > > > While it's more complicated, it's also more flexible.  You can
> > > > > communicate the panic message, whether the guest is attempting a
> > kdump
> > > > > and its own recovery or whether it wants the host to do it, etc., you
> > > > > can communicate less severe failures like oopses.
> > > > >
> > > > hypercall can take arguments to achieve the same.
> > >
> > > It has to be designed in advance; and every time we notice something's
> > > missing we have to update the host kernel.
> > >
> >
> > We and in the designed stage now. Not to late to design something flexible
> > :) Panic hypercall can take GPA of a buffer where host puts panic info
> > as a parameter.  This buffer can be read by QEMU and passed to management.
> >
> 
> If a host kernel change is in the works, I think it might be cleanest to
> have the host kernel export a new kind of VCPU exit for unhandled-by-KVM
> hypercalls.  Then usermode can respond to the hypercall as appropriate.
>  This would permit adding or changing future hypercalls without host kernel
> changes.
> 
There was such vm exit (KVM_EXIT_HYPERCALL), but it was deemed to be a
bad idea.

> "Guest panic" is almost the definition of not-a-fast-path, and so what's
> the reason to handle it in the host kernel.
The only "handling" that kernel does is maps hypercall to panic exit.

> 
> Punting to user-space wouldn't be a magic bullet for getting good
> interfaces designed, but in my opinion it is a better place to be doing
> them.

--
                        Gleb.