[PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set

[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.

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>
---
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