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

[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@xxxxxxxxxx>
Cc: "Radim KrÄmÃÅ" <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>
---
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