Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

Peter Zijlstra
2020-08-11T07:41:54Z

On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:

> My hypothesis here is simply that kvm_wait() may be called in a place
> where we get the same case I mentioned to Peter,
>
> raw_local_irq_save(); /* or other IRQs off without tracing */
> ...
> kvm_wait() /* IRQ state tracing gets confused */
> ...
> raw_local_irq_restore();
>
> and therefore, using raw variants in kvm_wait() works. It's also safe
> because it doesn't call any other libraries that would result in corrupt

Yes, this is definitely an issue.

Tracing, we also musn't call into tracing when using raw_local_irq_*().
Because then we re-intoduce this same issue all over again.

Both halt() and safe_halt() are more paravirt calls, but given we're in
a KVM paravirt call already, I suppose we can directly use native_*()
here.

Something like so then... I suppose, but then the Xen variants need TLC
too.

---
arch/x86/include/asm/irqflags.h | 4 ++--
arch/x86/include/asm/kvm_para.h | 18 +++++++++---------
arch/x86/kernel/kvm.c | 14 +++++++-------
3 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index 02a0cf547d7b..7c614db25274 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -54,13 +54,13 @@ static __always_inline void native_irq_enable(void)
asm volatile("sti": : :"memory");
}

-static inline __cpuidle void native_safe_halt(void)
+static __always_inline __cpuidle void native_safe_halt(void)
{
mds_idle_clear_cpu_buffers();
asm volatile("sti; hlt": : :"memory");
}

-static inline __cpuidle void native_halt(void)
+static __always_inline __cpuidle void native_halt(void)
{
mds_idle_clear_cpu_buffers();
asm volatile("hlt": : :"memory");
diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 49d3a9edb06f..90f7ea58ebb0 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -30,7 +30,7 @@ static inline bool kvm_check_and_clear_guest_paused(void)
* noted by the particular hypercall.
*/

-static inline long kvm_hypercall0(unsigned int nr)
+static __always_inline long kvm_hypercall0(unsigned int nr)
{
long ret;
asm volatile(KVM_HYPERCALL
@@ -40,7 +40,7 @@ static inline long kvm_hypercall0(unsigned int nr)
return ret;
}

-static inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
+static __always_inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
{
long ret;
asm volatile(KVM_HYPERCALL
@@ -50,8 +50,8 @@ static inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
return ret;
}

-static inline long kvm_hypercall2(unsigned int nr, unsigned long p1,
- unsigned long p2)
+static __always_inline long kvm_hypercall2(unsigned int nr, unsigned long p1,
+ unsigned long p2)
{
long ret;
asm volatile(KVM_HYPERCALL
@@ -61,8 +61,8 @@ static inline long kvm_hypercall2(unsigned int nr, unsigned long p1,
return ret;
}

-static inline long kvm_hypercall3(unsigned int nr, unsigned long p1,
- unsigned long p2, unsigned long p3)
+static __always_inline long kvm_hypercall3(unsigned int nr, unsigned long p1,
+ unsigned long p2, unsigned long p3)
{
long ret;
asm volatile(KVM_HYPERCALL
@@ -72,9 +72,9 @@ static inline long kvm_hypercall3(unsigned int nr, unsigned long p1,
return ret;
}

-static inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
- unsigned long p2, unsigned long p3,
- unsigned long p4)
+static __always_inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
+ unsigned long p2, unsigned long p3,
+ unsigned long p4)
{
long ret;
asm volatile(KVM_HYPERCALL
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 233c77d056c9..15f8dfd8812d 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -779,7 +779,7 @@ arch_initcall(kvm_alloc_cpumask);
#ifdef CONFIG_PARAVIRT_SPINLOCKS

/* Kick a cpu by its apicid. Used to wake up a halted vcpu */
-static void kvm_kick_cpu(int cpu)
+static notrace kvm_kick_cpu(int cpu)
{
int apicid;
unsigned long flags = 0;
@@ -790,14 +790,14 @@ static void kvm_kick_cpu(int cpu)

#include <asm/qspinlock.h>

-static void kvm_wait(u8 *ptr, u8 val)
+static notrace kvm_wait(u8 *ptr, u8 val)
{
unsigned long flags;

if (in_nmi())
return;

- local_irq_save(flags);
+ raw_local_irq_save(flags);

if (READ_ONCE(*ptr) != val)
goto out;
@@ -808,16 +808,16 @@ static void kvm_wait(u8 *ptr, u8 val)
* in irq spinlock slowpath and no spurious interrupt occur to save us.
*/
if (arch_irqs_disabled_flags(flags))
- halt();
+ native_halt();
else
- safe_halt();
+ native_safe_halt();

out:
- local_irq_restore(flags);
+ raw_local_irq_restore(flags);
}

#ifdef CONFIG_X86_32
-__visible bool __kvm_vcpu_is_preempted(long cpu)
+__visible notrace bool __kvm_vcpu_is_preempted(long cpu)
{
struct kvm_steal_time *src = &per_cpu(steal_time, cpu);