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Date:   Thu, 2 Nov 2017 11:08:54 -0700
From:   Eduardo Valentin <eduval@...zon.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     Eduardo Valentin <eduval@...zon.com>, <rkrcmar@...hat.com>,
        Matt Wilson <msw@...zon.com>, Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>, <kvm@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Jan H . Schoenherr" <jschoenh@...zon.de>,
        Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

On Thu, Nov 02, 2017 at 06:56:46PM +0100, Paolo Bonzini wrote:
> On 02/11/2017 18:45, Eduardo Valentin wrote:
> > Currently, the existing qspinlock implementation will fallback to
> > test-and-set if the hypervisor has not set the PV_UNHALT flag.
> > 
> > This patch gives the opportunity to guest kernels to select
> > between test-and-set and the regular queueu fair lock implementation
> > based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
> > flag is not set, the code will still fall back to test-and-set,
> > but when the PV_DEDICATED flag is set, the code will use
> > the regular queue spinlock implementation.
> 
> Have you seen Waiman's series that lets you specify this on the guest
> command line instead?  Would this be acceptable for your use case?
> 

No, can you please share a link to it? is it already merged to tip/master?

> (In other words, is there a difference for you between making the host
> vs. guest administrator toggle the feature?  "@amazon.com" means you are
> the host admin, how would you use it?)
> 

The way I think of this is this is a flag set by host side so the guest adapts accordingly.

If the admin in guest side wants to ignore what the host is flagging, that is a different story.

> Thanks,
> 
> Paolo
> 
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> > Cc: Jonathan Corbet <corbet@....net>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: x86@...nel.org
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Waiman Long <longman@...hat.com>
> > Cc: kvm@...r.kernel.org
> > Cc: linux-doc@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > Cc: Jan H. Schoenherr <jschoenh@...zon.de>
> > Cc: Anthony Liguori <aliguori@...zon.com>
> > Suggested-by: Matt Wilson <msw@...zon.com>
> > Signed-off-by: Eduardo Valentin <eduval@...zon.com>
> > ---
> > V2:
> >  - rebase on top of tip/master
> > 
> >  Documentation/virtual/kvm/cpuid.txt  | 6 ++++++
> >  arch/x86/include/asm/qspinlock.h     | 4 ++++
> >  arch/x86/include/uapi/asm/kvm_para.h | 1 +
> >  3 files changed, 11 insertions(+)
> > 
> > diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> > index 3c65feb..117066a 100644
> > --- a/Documentation/virtual/kvm/cpuid.txt
> > +++ b/Documentation/virtual/kvm/cpuid.txt
> > @@ -54,6 +54,12 @@ KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
> >                                     ||       || before enabling paravirtualized
> >                                     ||       || spinlock support.
> >  ------------------------------------------------------------------------------
> > +KVM_FEATURE_PV_DEDICATED           ||     8 || guest checks this feature bit
> > +                                   ||       || to determine if they run on
> > +                                   ||       || dedicated vCPUs, allowing opti-
> > +                                   ||       || mizations such as usage of
> > +                                   ||       || qspinlocks.
> > +------------------------------------------------------------------------------
> >  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
> >                                     ||       || per-cpu warps are expected in
> >                                     ||       || kvmclock.
> > diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
> > index 308dfd0..3751898 100644
> > --- a/arch/x86/include/asm/qspinlock.h
> > +++ b/arch/x86/include/asm/qspinlock.h
> > @@ -2,6 +2,8 @@
> >  #define _ASM_X86_QSPINLOCK_H
> >  
> >  #include <linux/jump_label.h>
> > +#include <linux/kvm_para.h>
> > +
> >  #include <asm/cpufeature.h>
> >  #include <asm-generic/qspinlock_types.h>
> >  #include <asm/paravirt.h>
> > @@ -57,6 +59,8 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
> >  	if (!static_branch_likely(&virt_spin_lock_key))
> >  		return false;
> >  
> > +	if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED))
> > +		return false;
> >  	/*
> >  	 * On hypervisors without PARAVIRT_SPINLOCKS support we fall
> >  	 * back to a Test-and-Set spinlock, because fair locks have
> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> > index a965e5b0..d151300 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -24,6 +24,7 @@
> >  #define KVM_FEATURE_STEAL_TIME		5
> >  #define KVM_FEATURE_PV_EOI		6
> >  #define KVM_FEATURE_PV_UNHALT		7
> > +#define KVM_FEATURE_PV_DEDICATED	8
> >  
> >  /* The last 8 bits are used to indicate how to interpret the flags field
> >   * in pvclock structure. If no bits are set, all flags are ignored.
> > 
> 
> 

-- 
All the best,
Eduardo Valentin

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