Re: Hypercall for sleeping for some amount of time?

[Stefan Fritsch <sf@sfritsch.de>]

On Friday 23 January 2015 10:53:00, Paolo Bonzini wrote:
> On 23/01/2015 10:40, Stefan Fritsch wrote:
> > Hi,
> > 
> > there are situations where a guest needs to wait, but has
> > interrupts disabled.  Usually this is done by polling in a tight
> > loop, which is a waste of CPU cycles on a virtualized system.
> > Therefore I propose to add a hypercall to KVM that allows the
> > guest to yield the CPU for a specified amount of time.
> > 
> > One significant use case is the guest sitting in a kernel debugger
> > and polling for user input. But I expect that there are other
> > situations where such a hypercall could be useful, for example
> > during hardware detection at startup.
> > 
> > What do you think about this idea?
> > 
> > What would be the preferred interface for such a hypercall? Using
> > vmcall like KVM_HC_*, or MSR write, or even a simple IO port
> > write? Is there a way to make this architecture independent?
> > 
> > For detection, at least a new bit in KVM_CPUID_FEATURES would be
> > necessary.  Does one also need more information, like the minimum
> > supported delay and the expected granularity of the delay? Also,
> > should the call be simply "best effort" or should it guarantee
> > that it sleeps for at least the requested time? I think keeping
> > it simple at first would be best.
> 
> I think a platform device similar to drivers/platform/x86/pvpanic.c
> would be best.  That would be a simple IO port write
> (milliseconds?), with the port described in ACPI.

Wasn't there some issue that Windows guests would then ask for a 
driver? AIUI, pvpanic has been disabled by default for this reason. 
Also, it would be nice if simplistic bootloaders without ACPI support 
would also be able to use the feature.

Cheers,
Stefan