Re: [RFC PATCH v3] Add rcu user eqs exception hooks for async page fault

[Li Zhong <zhong@linux.vnet.ibm.com>]

On Fri, 2012-11-30 at 12:26 +0200, Gleb Natapov wrote:
> On Fri, Nov 30, 2012 at 05:18:41PM +0800, Li Zhong wrote:
> > This patch adds user eqs exception hooks for async page fault page not
> > present code path, to exit the user eqs and re-enter it as necessary. 
> > 
> > Async page fault is different from other exceptions that it may be
> > triggered from idle process, so we still need rcu_irq_enter() and
> > rcu_irq_exit() to exit cpu idle eqs when needed, to protect the code
> > that needs use rcu.
> > 
> > As Frederic pointed out it would be safest and simplest to protect the
> > whole kvm_async_pf_task_wait(). Otherwise, "we need to check all the
> > code there deeply for potential RCU uses and ensure it will never be
> > extended later to use RCU.".
> > 
> > However, We'd better re-enter the cpu idle eqs if we get the exception
> > in cpu idle eqs, by calling rcu_irq_exit() before native_safe_halt(). 
> > 
> > So the patch does what Frederic suggested for rcu_irq_*() API usage
> > here, except that I moved the rcu_irq_*() pair originally in
> > do_async_page_fault() into kvm_async_pf_task_wait(). 
> > 
> > That's because, I think it's better to have rcu_irq_*() pairs to be in
> > one function ( rcu_irq_exit() after rcu_irq_enter() ), especially here,
> > kvm_async_pf_task_wait() has other callers, which might cause
> > rcu_irq_exit() be called without a matching rcu_irq_enter() before it,
> > which is illegal if the cpu happens to be in rcu idle state. 
> > 
> > Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
> > ---
> >  arch/x86/kernel/kvm.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 4180a87..342b00b 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -42,6 +42,7 @@
> >  #include <asm/apic.h>
> >  #include <asm/apicdef.h>
> >  #include <asm/hypervisor.h>
> > +#include <asm/rcu.h>
> >  
> >  static int kvmapf = 1;
> >  
> > @@ -112,6 +113,8 @@ void kvm_async_pf_task_wait(u32 token)
> >  	DEFINE_WAIT(wait);
> >  	int cpu, idle;
> >  
> > +	rcu_irq_enter();
> > +
> Why move rcu_irq_*() calls into kvm_async_pf_task_wait()?

I think it is not good for a function to have a rcu_irq_exit(), which
needs a matching rcu_irq_enter() in its caller. 

Here, if not move rcu_irq_* in, then the rcu_irq_exit() before
native_safe_halt() in kvm_async_pf_task_wait() is the one that needs the
matching rcu_irq_enter() in do_async_page_fault(). And, for this case,
kvm_async_pf_task_wait() even has other caller - pf_interception().
Maybe it will always be rcu non-idle for pf_interception (so a matching
rcu_irq_enter() is not needed), or maybe we could (or need) add
rcu_irq_*() in pf_interception().  But I still think it's good to have
those function calls that need to be matched be contained in one
function.

Thanks, Zhong

> >  	cpu = get_cpu();
> >  	idle = idle_cpu(cpu);
> >  	put_cpu();
> > @@ -123,6 +126,8 @@ void kvm_async_pf_task_wait(u32 token)
> >  		hlist_del(&e->link);
> >  		kfree(e);
> >  		spin_unlock(&b->lock);
> > +
> > +		rcu_irq_exit();
> We can skip that if  rcu_irq_*() will stay outside.
> 
> >  		return;
> >  	}
> >  
> > @@ -147,13 +152,16 @@ void kvm_async_pf_task_wait(u32 token)
> >  			/*
> >  			 * We cannot reschedule. So halt.
> >  			 */
> > +			rcu_irq_exit();
> >  			native_safe_halt();
> > +			rcu_irq_enter();
> >  			local_irq_disable();
> >  		}
> >  	}
> >  	if (!n.halted)
> >  		finish_wait(&n.wq, &wait);
> >  
> > +	rcu_irq_exit();
> >  	return;
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
> > @@ -247,10 +255,10 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
> >  		break;
> >  	case KVM_PV_REASON_PAGE_NOT_PRESENT:
> >  		/* page is swapped out by the host. */
> > -		rcu_irq_enter();
> > +		exception_enter(regs);
> >  		exit_idle();
> >  		kvm_async_pf_task_wait((u32)read_cr2());
> > -		rcu_irq_exit();
> > +		exception_exit(regs);
> >  		break;
> >  	case KVM_PV_REASON_PAGE_READY:
> >  		rcu_irq_enter();
> > -- 
> > 1.7.11.4
> 
> --
> 			Gleb.
>