Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-11-06 12:26-0800, Eduardo Valentin:
> 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;

Hm, every spinlock slowpath calls cpuid, which causes a VM exit, so I
wouldn't expect it to be faster than the existing implementations.
(Using the static key would be better.)

How does this patch perform compared to user-forced qspinlock and hybrid
pvqspinlock?

Thanks.