[PATCH v2] xen/events: remove event handling recursion detection

[PATCH v2] xen/events: remove event handling recursion detection

[Juergen Gross]

__xen_evtchn_do_upcall() contains guards against being called
recursively. This mechanism was introduced in the early pvops times
(kernel 2.6.26) when there were all the Xen backend drivers missing
from the upstream kernel, and some of those out-of-tree drivers were
enabling interrupts in their event handlers (which was explicitly
allowed in the initial XenoLinux).

Nowadays we don't need to support those old drivers any more and the
capability to allow recursive calls of __xen_evtchn_do_upcall() can
be removed.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2: adapt commit message (Jan Beulich)
---
 drivers/xen/events/events_base.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
index 6c8843968a52..33212c494afd 100644
--- a/drivers/xen/events/events_base.c
+++ b/drivers/xen/events/events_base.c
@@ -1213,31 +1213,21 @@ void xen_send_IPI_one(unsigned int cpu, enum ipi_vector vector)
 	notify_remote_via_irq(irq);
 }
 
-static DEFINE_PER_CPU(unsigned, xed_nesting_count);
-
 static void __xen_evtchn_do_upcall(void)
 {
 	struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
-	int cpu = get_cpu();
-	unsigned count;
+	int cpu = smp_processor_id();
 
 	do {
 		vcpu_info->evtchn_upcall_pending = 0;
 
-		if (__this_cpu_inc_return(xed_nesting_count) - 1)
-			goto out;
-
 		xen_evtchn_handle_events(cpu);
 
 		BUG_ON(!irqs_disabled());
 
-		count = __this_cpu_read(xed_nesting_count);
-		__this_cpu_write(xed_nesting_count, 0);
-	} while (count != 1 || vcpu_info->evtchn_upcall_pending);
-
-out:
+		rmb(); /* Hypervisor can set upcall pending. */
 
-	put_cpu();
+	} while (vcpu_info->evtchn_upcall_pending);
 }
 
 void xen_evtchn_do_upcall(struct pt_regs *regs)
-- 
2.16.4

Re: [PATCH v2] xen/events: remove event handling recursion detection

[Boris Ostrovsky]

On 11/28/19 3:45 AM, Juergen Gross wrote:
> -
>  static void __xen_evtchn_do_upcall(void)
>  {
>  	struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
> -	int cpu = get_cpu();
> -	unsigned count;
> +	int cpu = smp_processor_id();
>  
>  	do {
>  		vcpu_info->evtchn_upcall_pending = 0;
>  
> -		if (__this_cpu_inc_return(xed_nesting_count) - 1)
> -			goto out;
> -
>  		xen_evtchn_handle_events(cpu);
>  
>  		BUG_ON(!irqs_disabled());
>  
> -		count = __this_cpu_read(xed_nesting_count);
> -		__this_cpu_write(xed_nesting_count, 0);
> -	} while (count != 1 || vcpu_info->evtchn_upcall_pending);
> -
> -out:
> +		rmb(); /* Hypervisor can set upcall pending. */

virt_rmb() perhaps then?

-boris

>  
> -	put_cpu();
> +	} while (vcpu_info->evtchn_upcall_pending);
>  }
>  
>  void xen_evtchn_do_upcall(struct pt_regs *regs)

Re: [PATCH v2] xen/events: remove event handling recursion detection

[Jürgen Groß]

On 28.11.19 22:37, Boris Ostrovsky wrote:
> On 11/28/19 3:45 AM, Juergen Gross wrote:
>> -
>>   static void __xen_evtchn_do_upcall(void)
>>   {
>>   	struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
>> -	int cpu = get_cpu();
>> -	unsigned count;
>> +	int cpu = smp_processor_id();
>>   
>>   	do {
>>   		vcpu_info->evtchn_upcall_pending = 0;
>>   
>> -		if (__this_cpu_inc_return(xed_nesting_count) - 1)
>> -			goto out;
>> -
>>   		xen_evtchn_handle_events(cpu);
>>   
>>   		BUG_ON(!irqs_disabled());
>>   
>> -		count = __this_cpu_read(xed_nesting_count);
>> -		__this_cpu_write(xed_nesting_count, 0);
>> -	} while (count != 1 || vcpu_info->evtchn_upcall_pending);
>> -
>> -out:
>> +		rmb(); /* Hypervisor can set upcall pending. */
> 
> virt_rmb() perhaps then?

Yes, that's better.


Juergen