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

From: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	KVM <kvm@vger.kernel.org>,
	linux-doc@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Virtualization <virtualization@lists.linux-foundation.org>,
	Paul Mackerras <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
	Xen <xen-devel@lists.xensource.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Glauber Costa <glommer@redhat.com>, X86 <x86@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Avi Kivity <avi@redhat.com>,
	Rik van Riel <riel@redhat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Sasha Levin <levinsasha928@gmail.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Greg Kroah-Hartman <gregkh@suse.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Dave Hansen <dave
Subject: Re: [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock
Date: Tue, 17 Jan 2012 18:41:03 +0530	[thread overview]
Message-ID: <20120117131103.GD30398@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120117125126.GQ2167@redhat.com>

* Gleb Natapov <gleb@redhat.com> [2012-01-17 14:51:26]:

> On Tue, Jan 17, 2012 at 05:56:50PM +0530, Srivatsa Vaddagiri wrote:
> > * Gleb Natapov <gleb@redhat.com> [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.

> 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).

- vatsa