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Date:   Thu, 9 Nov 2017 17:05:36 +0100
From:   Radim Krcmar <rkrcmar@redhat.com>
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     Wanpeng Li <kernellwp@gmail.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Paolo Bonzini <pbonzini@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>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
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2017-11-09 10:53-0500, Pankaj Gupta:
> 
> 
> > 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.

Userspace can already configure it through KVM_SET_CPUID2 ioctl
(regardless of KVM version).

> 2] PV TLB should also behave as per option PV_DEDICATED for better performance.

Right,

thanks.

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