Re: [Xen-ia64-devel] [PATCH] pal_halt_light emulate for domU TAKE3

[Isaku Yamahata <yamahata@xxxxxxxxxxxxx>]

Hi Atsuhi.

On Mon, Aug 21, 2006 at 08:38:33PM +0900, Atsushi SAKAI wrote:

> 2)The VIRQ_ITC cycle is destroyed 
>   in case vcpu migration occurred when vcpu_pend_timer runs (called by 
> hlt_timer_fn)
>   
>   The VIRQ_ITC cycle is (simple writing)
>    ia64_get_itc reaches domain_itm at Xen.
>    send VIRQ_ITC to GuestOS
>    GuestOS handles and set  next itm by hypercall to Xen
>    Repeats.
>    
>   
>   Currently, the function order in vcpu_pend_timer is 
>   a)send VIRQ_ITC(vcpu_pend_timer)
>   b)then stamp the signal send information.(update domain_itm_last)
>   In this configuration, 
>   domain_itm_last update time and hypercall set itm(which comes from VIRQ_ITC)
>   is important. 
>   
>   This order is problematic in case vcpu_migration occurred as follows
>   (following event occures nearly 1-2 hour running 
>   by 2xDomU 3vcpu(total 6vcpu for domU) in 4pcpu Linux Kernel Compile test)
>   
>    vcpu_pend_timer@A is started.(GuestOS is not running at this moment) 
>    interrupts@B
>    vcpu_pend_timer@A is paused
>    Credit Scheduler steal vcpu from A to B
>    vcpu_pend_timer@B is started and ended
>    
>    ******The following process should be after vcpu_pend_timer*****
>    domain_itm is set by GuestOS hypercall. and domain_itm updated.
>    
>    vcpu_pend_timer@A is resumed. but domain_itm is already updated.
> 
>    
>   To avoid this, order should be exchanged b) a)
>   After this fixation, we are free from timing consideration of 
>   VIRQ_ITC cycle and domain_itm_last update time.
>   (domain_itm_last update should be before the hypercall set itm)

Could you elaborate more? I don't understand.
What phenomenan did you observed?

vcpu_pend_timer() is called with its parameter as current.
It means that test_bit(_VCPUF_running, vcpu->vcpu_flags) is 
always true becuase vcpu == current.
On the other hand, scheduler mustn't steal vcpu with _VCPUF_running set.
(see __csched_vcpu_is_stealable())
The vcpu stealing doesn't occur so that the above sequence
doesn't make sense (to me at least.)


Thanks.
-- 
yamahata

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