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 Mon, Jan 16, 2012 at 07:41:17PM +0530, Srivatsa Vaddagiri wrote:
> * Avi Kivity <avi@xxxxxxxxxx> [2012-01-16 12:14:27]:
> 
> > > One option is to make the kick hypercall available only when
> > > yield_on_hlt=1?
> > 
> > It's not a good idea to tie various options together.  Features should
> > be orthogonal.
> > 
> > Can't we make it work?  Just have different handling for
> > KVM_REQ_PVLOCK_KICK (let 's rename it, and the hypercall, PV_UNHALT,
> > since we can use it for non-locks too).
> 
> 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().

> Having another hypercall to do yield/sleep (rather than effecting that
> via HLT) seems like an alternate clean solution here ..
> 
> - vatsa

--
                        Gleb.

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