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.
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Jan H. Schoenherr <jschoenh@xxxxxxxxx>
Cc: Anthony Liguori <aliguori@xxxxxxxxxx>
Suggested-by: Matt Wilson <msw@xxxxxxxxxx>
Signed-off-by: Eduardo Valentin <eduval@xxxxxxxxxx>
---
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.
--
2.7.4
