From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Eduardo Valentin <eduval@amazon.com>
Cc: 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>,
x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
Waiman Long <longman@redhat.com>,
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
Date: Wed, 8 Nov 2017 18:36:52 +0100 [thread overview]
Message-ID: <20171108173651.GA3664@flask> (raw)
In-Reply-To: <1509999980-10828-1-git-send-email-eduval@amazon.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.
next prev parent reply other threads:[~2017-11-08 17:37 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-06 20:26 [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set Eduardo Valentin
2017-11-07 12:23 ` Paolo Bonzini
2017-11-07 12:39 ` Eduardo Valentin
2017-11-07 12:43 ` Paolo Bonzini
2017-11-08 8:45 ` Eduardo Valentin
2017-11-08 17:36 ` Radim Krčmář [this message]
2017-11-09 8:55 ` Eduardo Valentin
2017-11-09 14:17 ` Radim Krčmář
2017-11-16 4:54 ` Eduardo Valentin
2017-11-27 0:43 ` Wanpeng Li
2017-11-09 12:43 ` Wanpeng Li
2017-11-09 15:53 ` Pankaj Gupta
2017-11-09 16:05 ` Radim Krcmar
2017-11-09 16:17 ` Peter Zijlstra
2017-11-09 16:37 ` Pankaj Gupta
2017-11-09 16:45 ` Radim Krcmar
2017-11-09 17:12 ` Peter Zijlstra
2017-11-09 17:15 ` Peter Zijlstra
2017-11-09 17:28 ` Peter Zijlstra
2017-11-09 17:35 ` Radim Krcmar
2017-11-10 2:07 ` Wanpeng Li
2017-11-10 7:59 ` Peter Zijlstra
2017-11-10 8:04 ` Wanpeng Li
2017-11-09 17:31 ` Radim Krcmar
2017-11-09 16:00 ` Radim Krcmar
2017-11-10 6:07 ` Wanpeng Li
2017-11-10 8:19 ` Paolo Bonzini
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