Re: [PATCH v2 for-4.14 2/3] xen/vm_event: add vm_event_check_pending_op

[Jan Beulich <jbeulich@suse.com>]

On 02.06.2020 13:47, Roger Pau Monné wrote:
> On Wed, May 20, 2020 at 08:31:53PM -0600, Tamas K Lengyel wrote:
>> Perform sanity checking when shutting vm_event down to determine whether
>> it is safe to do so. Error out with -EAGAIN in case pending operations
>> have been found for the domain.
>>
>> Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
>> ---
>>  xen/arch/x86/vm_event.c        | 23 +++++++++++++++++++++++
>>  xen/common/vm_event.c          | 17 ++++++++++++++---
>>  xen/include/asm-arm/vm_event.h |  7 +++++++
>>  xen/include/asm-x86/vm_event.h |  2 ++
>>  4 files changed, 46 insertions(+), 3 deletions(-)
>>
>> diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
>> index 848d69c1b0..a23aadc112 100644
>> --- a/xen/arch/x86/vm_event.c
>> +++ b/xen/arch/x86/vm_event.c
>> @@ -297,6 +297,29 @@ void vm_event_emulate_check(struct vcpu *v, vm_event_response_t *rsp)
>>      };
>>  }
>>  
>> +bool vm_event_check_pending_op(const struct vcpu *v)
>> +{
>> +    struct monitor_write_data *w = &v->arch.vm_event->write_data;
> 
> const
> 
>> +
>> +    if ( !v->arch.vm_event->sync_event )
>> +        return false;
>> +
>> +    if ( w->do_write.cr0 )
>> +        return true;
>> +    if ( w->do_write.cr3 )
>> +        return true;
>> +    if ( w->do_write.cr4 )
>> +        return true;
>> +    if ( w->do_write.msr )
>> +        return true;
>> +    if ( v->arch.vm_event->set_gprs )
>> +        return true;
>> +    if ( v->arch.vm_event->emulate_flags )
>> +        return true;
> 
> Can you please group this into a single if, ie:
> 
> if ( w->do_write.cr0 || w->do_write.cr3 || ... )
>     return true;
> 
>> +
>> +    return false;
>> +}
>> +
>>  /*
>>   * Local variables:
>>   * mode: C
>> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
>> index 127f2d58f1..2df327a42c 100644
>> --- a/xen/common/vm_event.c
>> +++ b/xen/common/vm_event.c
>> @@ -183,6 +183,7 @@ static int vm_event_disable(struct domain *d, struct vm_event_domain **p_ved)
>>      if ( vm_event_check_ring(ved) )
>>      {
>>          struct vcpu *v;
>> +        bool pending_op = false;
>>  
>>          spin_lock(&ved->lock);
>>  
>> @@ -192,9 +193,6 @@ static int vm_event_disable(struct domain *d, struct vm_event_domain **p_ved)
>>              return -EBUSY;
>>          }
>>  
>> -        /* Free domU's event channel and leave the other one unbound */
>> -        free_xen_event_channel(d, ved->xen_port);
>> -
>>          /* Unblock all vCPUs */
>>          for_each_vcpu ( d, v )
>>          {
>> @@ -203,8 +201,21 @@ static int vm_event_disable(struct domain *d, struct vm_event_domain **p_ved)
>>                  vcpu_unpause(v);
>>                  ved->blocked--;
>>              }
>> +
>> +            if ( vm_event_check_pending_op(v) )
>> +                pending_op = true;
> 
> You could just do:
> 
> pending_op |= vm_event_check_pending_op(v);
> 
> and avoid the initialization of pending_op above.

The initialization has to stay, or it couldn't be |= afaict.

> Or alternatively:
> 
> if ( !pending_op && vm_event_check_pending_op(v) )
>     pending_op = true;
> 
> Which avoid repeated calls to vm_event_check_pending_op when at least
> one vCPU is known to be busy.

    if ( !pending_op )
        pending_op = vm_event_check_pending_op(v);

?

Jan