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Date:   Thu, 9 Nov 2017 10:53:25 -0500 (EST)
From:   Pankaj Gupta <pagupta@redhat.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Eduardo Valentin <eduval@amazon.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Matt Wilson <msw@amazon.com>, Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Waiman Long <longman@redhat.com>, kvm <kvm@vger.kernel.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Jan H . Schoenherr" <jschoenh@amazon.de>,
        Anthony Liguori <aliguori@amazon.com>
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Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
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Thread-Topic: locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set
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> 2017-11-07 4:26 GMT+08:00 Eduardo Valentin <eduval@amazon.com>:
> > 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.
> >
> > With this patch, when in autoselect mode, the guest will
> > use the default spinlock implementation based on host feature
> > flags as follows:
> >
> > PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
> > PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock
> > PV_DEDICATED = 0, PV_UNHALT = 0: default is tas
> >
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: x86@kernel.org
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Waiman Long <longman@redhat.com>
> > Cc: kvm@vger.kernel.org
> > Cc: linux-doc@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: Jan H. Schoenherr <jschoenh@amazon.de>
> > Cc: Anthony Liguori <aliguori@amazon.com>
> > Suggested-by: Matt Wilson <msw@amazon.com>
> > Signed-off-by: Eduardo Valentin <eduval@amazon.com>
> > ---
> > V3:
> >  - When PV_DEDICATED is set (1), qspinlock is selected,
> >    regardless of the value of PV_UNHAULT. Suggested by Paolo Bonzini.
> >  - Refreshed on top of tip/master.
> > 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 +
> >  arch/x86/kernel/kvm.c                | 2 ++
> >  4 files changed, 13 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 5e16b5d..de42694 100644
> > --- a/arch/x86/include/asm/qspinlock.h
> > +++ b/arch/x86/include/asm/qspinlock.h
> > @@ -3,6 +3,8 @@
> >  #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>
> > @@ -58,6 +60,8 @@ 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 554aa8f..85a9875 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -25,6 +25,7 @@
> >  #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.
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 8bb9594..dacd7cf 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -642,6 +642,8 @@ void __init kvm_spinlock_init(void)
> >  {
> >         if (!kvm_para_available())
> >                 return;
> > +       if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED))
> > +               return;
> >         /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
> >         if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
> >                 return;
> > --
> > 2.7.4
> >
> 
> You should also add a cpuid flag in kvm part.

Also, I am thinking if PV_DEDICATED helps in performance and with conjunction
with PV TLB patch in other thread. For use-case e.g KVM-RT where we don't overcommit 
vCPU's and pin vCPU:pCPU 1:1 we need a way from host side with which user can decide 
to enable PV_DEDICATED option. Such that if vCPU's are unlikely going to preempt or 
sleep we should avoid traversing the cpulist in PV TLB code.

So, two things:

1] A way to configure PV_DEDICATED from host.
2] PV TLB should also behave as per option PV_DEDICATED for better performance.

Or I am missing any context here?
> 
> Regards,
> Wanpeng Li
> 

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