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

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Date:   Mon, 2 Oct 2017 17:11:07 -0400 (EDT)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v2] kvm/x86: Handle async PF in RCU read-side critical
 sections


----- Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 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();

Or even

 is_idle_task(current) ||
 (IS_ENABLED(CONFIG_PREEMPT_COUNT)
  ? preempt_count() > 1 || rcu_preempt_depth()
  : !user)

since PREEMPT selects PREEMPT_COUNT.

Paolo

> 							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:
> > > > 
> > > 
> 
>