Re: [PATCH v2] kvm/x86: Handle async PF in RCU read-side critical sections

[Paul E. McKenney]

On Mon, Oct 02, 2017 at 10:43:00PM +0800, Boqun Feng wrote:
> On Mon, Oct 02, 2017 at 01:41:03PM +0000, Paolo Bonzini wrote:
> [...]
> > > 
> > > Wanpeng, the callsite of kvm_async_pf_task_wait() in
> > > kvm_handle_page_fault() is for nested scenario, right? I take it we
> > > should handle it as if the fault happens when l1 guest is running in
> > > kernel mode, so @user should be 0, right?
> > 
> > In that case we can schedule, actually.  The guest will let another
> > process run.
> > 
> > In fact we could schedule safely most of the time in the
> > !user_mode(regs) case, it's just that with PREEMPT=n there's no
> > knowledge of whether we can do so.  This explains why we have never seen
> > the bug before.
> 
> Thanks, looks like I confused myself a little bit here. You are right.
> So in PREEMPT=n kernel, we only couldn't schedule when the async PF
> *interrupts* the *kernel*, while in the kvm_handle_page_fault(), we
> actually didn't interrupt the kernel, so it's fine.

Actually, we should be able to do a little bit better than that.
If PREEMPT=n but PREEMPT_COUNT=y, then preempt_count() will know
about RCU read-side critical sections via preempt_disable().

So maybe something like this?

	n.halted = is_idle_task(current) ||
		   preempt_count() > 1 ||
		   (!IS_ENABLED(CONFIG_PREEMPT) && !IS_ENABLED(CONFIG_PREEMPT_COUNT) && !user) ||
		   rcu_preempt_depth();

							Thanx, Paul

> > I had already applied v1, can you rebase and resend please?  Thanks,
> > 
> 
> Sure, I'm going to rename that parameter to "interrupt_kernel"(probably
> a bad name"), indicating whether the async PF interrupts the kernel.
> 
> But it's a little bit late today, will do that tomorrow.
> 
> Regards,
> Boqun
> 
> 
> > Paolo
> > 
> > >  arch/x86/include/asm/kvm_para.h | 4 ++--
> > >  arch/x86/kernel/kvm.c           | 9 ++++++---
> > >  arch/x86/kvm/mmu.c              | 2 +-
> > >  3 files changed, 9 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> > > index bc62e7cbf1b1..0a5ae6bb128b 100644
> > > --- a/arch/x86/include/asm/kvm_para.h
> > > +++ b/arch/x86/include/asm/kvm_para.h
> > > @@ -88,7 +88,7 @@ static inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
> > >  bool kvm_para_available(void);
> > >  unsigned int kvm_arch_para_features(void);
> > >  void __init kvm_guest_init(void);
> > > -void kvm_async_pf_task_wait(u32 token);
> > > +void kvm_async_pf_task_wait(u32 token, int user);
> > >  void kvm_async_pf_task_wake(u32 token);
> > >  u32 kvm_read_and_reset_pf_reason(void);
> > >  extern void kvm_disable_steal_time(void);
> > > @@ -103,7 +103,7 @@ static inline void kvm_spinlock_init(void)
> > >  
> > >  #else /* CONFIG_KVM_GUEST */
> > >  #define kvm_guest_init() do {} while (0)
> > > -#define kvm_async_pf_task_wait(T) do {} while(0)
> > > +#define kvm_async_pf_task_wait(T, U) do {} while(0)
> > >  #define kvm_async_pf_task_wake(T) do {} while(0)
> > >  
> > >  static inline bool kvm_para_available(void)
> > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > > index aa60a08b65b1..916f519e54c9 100644
> > > --- a/arch/x86/kernel/kvm.c
> > > +++ b/arch/x86/kernel/kvm.c
> > > @@ -117,7 +117,7 @@ static struct kvm_task_sleep_node *_find_apf_task(struct kvm_task_sleep_head *b,
> > >  	return NULL;
> > >  }
> > >  
> > > -void kvm_async_pf_task_wait(u32 token)
> > > +void kvm_async_pf_task_wait(u32 token, int user)
> > >  {
> > >  	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
> > >  	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
> > > @@ -140,7 +140,10 @@ void kvm_async_pf_task_wait(u32 token)
> > >  
> > >  	n.token = token;
> > >  	n.cpu = smp_processor_id();
> > > -	n.halted = is_idle_task(current) || preempt_count() > 1;
> > > +	n.halted = is_idle_task(current) ||
> > > +		   preempt_count() > 1 ||
> > > +		   (!IS_ENABLED(CONFIG_PREEMPT) && !user) ||
> > > +		   rcu_preempt_depth();
> > >  	init_swait_queue_head(&n.wq);
> > >  	hlist_add_head(&n.link, &b->list);
> > >  	raw_spin_unlock(&b->lock);
> > > @@ -268,7 +271,7 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
> > >  	case KVM_PV_REASON_PAGE_NOT_PRESENT:
> > >  		/* page is swapped out by the host. */
> > >  		prev_state = exception_enter();
> > > -		kvm_async_pf_task_wait((u32)read_cr2());
> > > +		kvm_async_pf_task_wait((u32)read_cr2(), user_mode(regs));
> > >  		exception_exit(prev_state);
> > >  		break;
> > >  	case KVM_PV_REASON_PAGE_READY:
> > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > > index eca30c1eb1d9..106d4a029a8a 100644
> > > --- a/arch/x86/kvm/mmu.c
> > > +++ b/arch/x86/kvm/mmu.c
> > > @@ -3837,7 +3837,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
> > >  	case KVM_PV_REASON_PAGE_NOT_PRESENT:
> > >  		vcpu->arch.apf.host_apf_reason = 0;
> > >  		local_irq_disable();
> > > -		kvm_async_pf_task_wait(fault_address);
> > > +		kvm_async_pf_task_wait(fault_address, 0);
> > >  		local_irq_enable();
> > >  		break;
> > >  	case KVM_PV_REASON_PAGE_READY:
> > > 
> >