Re: [Xen-devel] [PATCH V5 08/12] xen/vm_event: Check for VM_EVENT_FLAG_DUMMY only in Debug builds

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Fri, Feb 13, 2015 at 11:48 PM, Tamas K Lengyel
<tamas.lengyel@xxxxxxxxxxxx> wrote:
> On Fri, Feb 13, 2015 at 9:14 PM, Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 13/02/15 16:33, Tamas K Lengyel wrote:
>>> The flag is only used for debugging purposes, thus it should be only checked
>>> for in debug builds of Xen.
>>>
>>> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
>>
>> What is the purpose of the dummy flag?  I would have thought it would be
>> more dangerous to accidentally process a dummy response in a non-debug Xen.
>>
>> ~Andrew
>
> I honestly have no idea what was the real use-case for it. It is a way
> to signal to Xen to just remove the reponse from the ring without
> doing anything else with it so I figured it might be handy when
> debugging a toolstack.
>
> Tamas

As for processing it on a non-debug Xen: I think it's safe as the
response would just be handled according to the other flags that are
set in the response, like unpausing the vCPU that triggered the event.
IMHO its a wasteful check on production systems.

Tamas

>
>>
>>> ---
>>>  xen/arch/x86/mm/mem_sharing.c | 2 ++
>>>  xen/arch/x86/mm/p2m.c         | 2 ++
>>>  xen/common/mem_access.c       | 2 ++
>>>  3 files changed, 6 insertions(+)
>>>
>>> diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
>>> index 4e5477a..0459544 100644
>>> --- a/xen/arch/x86/mm/mem_sharing.c
>>> +++ b/xen/arch/x86/mm/mem_sharing.c
>>> @@ -606,8 +606,10 @@ int mem_sharing_sharing_resume(struct domain *d)
>>>              continue;
>>>          }
>>>
>>> +#ifndef NDEBUG
>>>          if ( rsp.flags & VM_EVENT_FLAG_DUMMY )
>>>              continue;
>>> +#endif
>>>
>>>          /* Validate the vcpu_id in the response. */
>>>          if ( (rsp.vcpu_id >= d->max_vcpus) || !d->vcpu[rsp.vcpu_id] )
>>> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
>>> index 5ce852e..68d57d7 100644
>>> --- a/xen/arch/x86/mm/p2m.c
>>> +++ b/xen/arch/x86/mm/p2m.c
>>> @@ -1312,8 +1312,10 @@ void p2m_mem_paging_resume(struct domain *d)
>>>              continue;
>>>          }
>>>
>>> +#ifndef NDEBUG
>>>          if ( rsp.flags & VM_EVENT_FLAG_DUMMY )
>>>              continue;
>>> +#endif
>>>
>>>          /* Validate the vcpu_id in the response. */
>>>          if ( (rsp.vcpu_id >= d->max_vcpus) || !d->vcpu[rsp.vcpu_id] )
>>> diff --git a/xen/common/mem_access.c b/xen/common/mem_access.c
>>> index a6d82d1..63f2b52 100644
>>> --- a/xen/common/mem_access.c
>>> +++ b/xen/common/mem_access.c
>>> @@ -44,8 +44,10 @@ void mem_access_resume(struct domain *d)
>>>              continue;
>>>          }
>>>
>>> +#ifndef NDEBUG
>>>          if ( rsp.flags & VM_EVENT_FLAG_DUMMY )
>>>              continue;
>>> +#endif
>>>
>>>          /* Validate the vcpu_id in the response. */
>>>          if ( (rsp.vcpu_id >= d->max_vcpus) || !d->vcpu[rsp.vcpu_id] )
>>

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