Re: [Xen-devel] [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock

[Gleb Natapov <gleb@xxxxxxxxxx>]

On Tue, Jan 17, 2012 at 06:41:03PM +0530, Srivatsa Vaddagiri wrote:
> * Gleb Natapov <gleb@xxxxxxxxxx> [2012-01-17 14:51:26]:
> 
> > On Tue, Jan 17, 2012 at 05:56:50PM +0530, Srivatsa Vaddagiri wrote:
> > > * Gleb Natapov <gleb@xxxxxxxxxx> [2012-01-17 11:14:13]:
> > > 
> > > > > The problem case I was thinking of was when guest VCPU would have 
> > > > > issued
> > > > > HLT with interrupts disabled. I guess one option is to inject an NMI,
> > > > > and have the guest kernel NMI handler recognize this and make
> > > > > adjustments such that the vcpu avoids going back to HLT instruction.
> > > > > 
> > > > Just kick vcpu out of a guest mode and adjust rip to point after HLT on
> > > > next re-entry. Don't forget to call vmx_clear_hlt().
> > > 
> > > Looks bit hackish to me compared to having another hypercall to yield!
> > > 
> > Do not see anything hackish about it. But what you described above (the
> > part I replied to) is not another hypercall, but yet another NMI source
> > and special handling in a guest.
> 
> True, which I didn't exactly like and hence was suggesting we use
> another hypercall to let spinning vcpu sleep.
> 
Ah, sorry. Missed that.

> > So what hypercall do you mean?
> 
> The hypercall is described below:
> 
> > > > > Having another hypercall to do yield/sleep (rather than effecting that
> > > > > via HLT) seems like an alternate clean solution here ..
> 
> and was implemented in an earlier version of this patch (v2) as
> KVM_HC_WAIT_FOR_KICK hypercall:
> 
> https://lkml.org/lkml/2011/10/23/211
> 
> Having the hypercall makes the intent of vcpu (to sleep on a kick) clear to 
> hypervisor vs assuming that because of a trapped HLT instruction (which
> will anyway won't work when yield_on_hlt=0).
> 
The purpose of yield_on_hlt=0 is to allow VCPU to occupy CPU for the
entire time slice no mater what. I do not think disabling yield on HLT
is even make sense in CPU oversubscribe scenario. Now if you'll call
KVM_HC_WAIT_FOR_KICK instead of HLT you will effectively ignore
yield_on_hlt=0 setting. This is like having PV HLT that does not obey
VMX exit control setting.

--
                        Gleb.

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