linux-kernel - Re: [PATCH RFC V2 3/5] kvm hypervisor : Add two hypercalls to support pv-ticketlock

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Message-ID: <4EA56385.9040302@redhat.com>
Date:	Mon, 24 Oct 2011 15:09:25 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>, Gleb Natapov <gleb@...hat.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	x86@...nel.org, KVM <kvm@...r.kernel.org>,
	Dave Jiang <dave.jiang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Xen <xen-devel@...ts.xensource.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Suzuki Poulose <suzuki@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC V2 3/5] kvm hypervisor : Add two hypercalls to support
 pv-ticketlock

On 10/24/2011 02:27 PM, Srivatsa Vaddagiri wrote:
> Good point. Assuming yield_on_hlt=1, that would allow the vcpu to be put
> to sleep and let other vcpus make progress.
>
> I guess with that change, we can also dropthe need for other hypercall
> introduced in this patch (kvm_pv_kick_cpu_op()). Essentially a vcpu sleeping 
> because of HLT instruction can be woken up by a IPI issued by vcpu releasing a
> lock.

Not if interrupts are disabled.  My original plan was to use NMIs for
wakeups, but it turns out NMIs can be coalesced under certain rare
circumstances; this requires workarounds by the generic NMI code that
make NMIs too slow.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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