Re: [PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check

Peter Zijlstra
2016-11-15T15:47:06Z

On Wed, Nov 02, 2016 at 05:08:33AM -0400, Pan Xinhui wrote:
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index 0f400c0..38c3bb7 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -310,6 +310,8 @@ struct pv_lock_ops {
>
> void (*wait)(u8 *ptr, u8 val);
> void (*kick)(int cpu);
> +
> + bool (*vcpu_is_preempted)(int cpu);
> };

So that ends up with a full function call in the native case. I did
something like the below on top, completely untested, not been near a
compiler etc..

It doesn't get rid of the branch, but at least it avoids the function
call, and hardware should have no trouble predicting a constant
condition.

Also, it looks like you end up not setting vcpu_is_preempted when KVM
doesn't support steal clock, which would end up in an instant NULL
deref. Fixed that too.

---
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -673,6 +673,11 @@ static __always_inline void pv_kick(int
PVOP_VCALL1(pv_lock_ops.kick, cpu);
}

+static __always_inline void pv_vcpu_is_prempted(int cpu)
+{
+ PVOP_VCALLEE1(pv_lock_ops.vcpu_is_preempted, cpu);
+}
+
#endif /* SMP && PARAVIRT_SPINLOCKS */

#ifdef CONFIG_X86_32
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -309,7 +309,7 @@ struct pv_lock_ops {
void (*wait)(u8 *ptr, u8 val);
void (*kick)(int cpu);

- bool (*vcpu_is_preempted)(int cpu);
+ struct paravirt_callee_save vcpu_is_preempted;
};

/* This contains all the paravirt structures: we get a convenient
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -32,6 +32,12 @@ static inline void queued_spin_unlock(st
{
pv_queued_spin_unlock(lock);
}
+
+#define vcpu_is_preempted vcpu_is_preempted
+static inline bool vcpu_is_preempted(int cpu)
+{
+ return pv_vcpu_is_preempted(cpu);
+}
#else
static inline void queued_spin_unlock(struct qspinlock *lock)
{
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -26,14 +26,6 @@
extern struct static_key paravirt_ticketlocks_enabled;
static __always_inline bool static_key_false(struct static_key *key);

-#ifdef CONFIG_PARAVIRT_SPINLOCKS
-#define vcpu_is_preempted vcpu_is_preempted
-static inline bool vcpu_is_preempted(int cpu)
-{
- return pv_lock_ops.vcpu_is_preempted(cpu);
-}
-#endif
-
#include <asm/qspinlock.h>

/*
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -415,15 +415,6 @@ void kvm_disable_steal_time(void)
wrmsr(MSR_KVM_STEAL_TIME, 0, 0);
}

-static bool kvm_vcpu_is_preempted(int cpu)
-{
- struct kvm_steal_time *src;
-
- src = &per_cpu(steal_time, cpu);
-
- return !!src->preempted;
-}
-
#ifdef CONFIG_SMP
static void __init kvm_smp_prepare_boot_cpu(void)
{
@@ -480,9 +471,6 @@ void __init kvm_guest_init(void)
if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
has_steal_clock = 1;
pv_time_ops.steal_clock = kvm_steal_clock;
-#ifdef CONFIG_PARAVIRT_SPINLOCKS
- pv_lock_ops.vcpu_is_preempted = kvm_vcpu_is_preempted;
-#endif
}

if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
@@ -604,6 +592,14 @@ static void kvm_wait(u8 *ptr, u8 val)
local_irq_restore(flags);
}

+static bool __kvm_vcpu_is_preempted(int cpu)
+{
+ struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
+
+ return !!src->preempted;
+}
+PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
+
/*
* Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
*/
@@ -620,6 +616,12 @@ void __init kvm_spinlock_init(void)
pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
pv_lock_ops.wait = kvm_wait;
pv_lock_ops.kick = kvm_kick_cpu;
+ pv_lock_ops.vcpu_is_preempted = PV_CALLEE_SAVE(__native_vcpu_is_preempted);
+
+ if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
+ pv_lock_ops.vcpu_is_preempted =
+ PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
+ }
}

static __init int kvm_spinlock_init_jump(void)
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -12,7 +12,6 @@ __visible void __native_queued_spin_unlo
{
native_queued_spin_unlock(lock);
}
-
PV_CALLEE_SAVE_REGS_THUNK(__native_queued_spin_unlock);

bool pv_is_native_spin_unlock(void)
@@ -21,9 +20,16 @@ bool pv_is_native_spin_unlock(void)
__raw_callee_save___native_queued_spin_unlock;
}

-static bool native_vcpu_is_preempted(int cpu)
+__visible bool __native_vcpu_is_preempted(int cpu)
{
- return 0;
+ return false;
+}
+PV_CALLEE_SAVE_REGS_THUNK(__native_vcpu_is_preempted);
+
+bool pv_is_native_vcpu_is_preempted(void)
+{
+ return pv_lock_ops.queued_spin_unlock.func ==
+ __raw_callee_save__native_vcpu_is_preempted;
}

struct pv_lock_ops pv_lock_ops = {
@@ -32,7 +38,7 @@ struct pv_lock_ops pv_lock_ops = {
.queued_spin_unlock = PV_CALLEE_SAVE(__native_queued_spin_unlock),
.wait = paravirt_nop,
.kick = paravirt_nop,
- .vcpu_is_preempted = native_vcpu_is_preempted,
+ .vcpu_is_preempted = PV_CALLEE_SAVE(__native_vcpu_is_preempted),
#endif /* SMP */
};
EXPORT_SYMBOL(pv_lock_ops);
--- a/arch/x86/kernel/paravirt_patch_32.c
+++ b/arch/x86/kernel/paravirt_patch_32.c
@@ -11,6 +11,7 @@ DEF_NATIVE(pv_mmu_ops, read_cr3, "mov %c

#if defined(CONFIG_PARAVIRT_SPINLOCKS)
DEF_NATIVE(pv_lock_ops, queued_spin_unlock, "movb $0, (%eax)");
+DEF_NATIVE(pv_lock_ops, vcpu_is_preempted, "movl $0, %eax");
#endif

unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
@@ -26,6 +27,7 @@ unsigned paravirt_patch_ident_64(void *i
}

extern bool pv_is_native_spin_unlock(void);
+extern bool pv_is_native_vcpu_is_preempted(void);

unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
unsigned long addr, unsigned len)
@@ -54,6 +56,12 @@ unsigned native_patch(u8 type, u16 clobb
end = end_pv_lock_ops_queued_spin_unlock;
goto patch_site;
}
+ case PARAVIRT_PATCH(pv_lock_ops.vcpu_is_preempted):
+ if (pv_is_native_vcpu_is_preempted()) {
+ start = start_pv_lock_ops_vcpu_is_preempted;
+ end = end_pv_lock_ops_vcpu_is_preempted;
+ goto patch_site;
+ }
#endif

default:
--- a/arch/x86/kernel/paravirt_patch_64.c
+++ b/arch/x86/kernel/paravirt_patch_64.c
@@ -20,6 +20,7 @@ DEF_NATIVE(, mov64, "mov %rdi, %rax");

#if defined(CONFIG_PARAVIRT_SPINLOCKS)
DEF_NATIVE(pv_lock_ops, queued_spin_unlock, "movb $0, (%rdi)");
+DEF_NATIVE(pv_lock_ops, vcpu_is_preempted, "movl $0, rax");
#endif

unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
@@ -35,6 +36,7 @@ unsigned paravirt_patch_ident_64(void *i
}

extern bool pv_is_native_spin_unlock(void);
+extern bool pv_is_native_vcpu_is_preempted(void);

unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
unsigned long addr, unsigned len)
@@ -66,6 +68,12 @@ unsigned native_patch(u8 type, u16 clobb
end = end_pv_lock_ops_queued_spin_unlock;
goto patch_site;
}
+ case PARAVIRT_PATCH(pv_lock_ops.vcpu_is_preempted):
+ if (pv_is_native_vcpu_is_preempted()) {
+ start = start_pv_lock_ops_vcpu_is_preempted;
+ end = end_pv_lock_ops_vcpu_is_preempted;
+ goto patch_site;
+ }
#endif

default:
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -114,6 +114,8 @@ void xen_uninit_lock_cpu(int cpu)
per_cpu(irq_name, cpu) = NULL;
}

+PV_CALLEE_SAVE_REGS_THUNK(xen_vcpu_stolen);
+
/*
* Our init of PV spinlocks is split in two init functions due to us
* using paravirt patching and jump labels patching and having to do
@@ -136,8 +138,7 @@ void __init xen_init_spinlocks(void)
pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
pv_lock_ops.wait = xen_qlock_wait;
pv_lock_ops.kick = xen_qlock_kick;
-
- pv_lock_ops.vcpu_is_preempted = xen_vcpu_stolen;
+ pv_lock_ops.vcpu_is_preempted = PV_CALLEE_SAVE(xen_vcpu_stolen);
}

/*