Re: [Xen-devel] [PATCH 1/3] xen: drop in_atomic()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.05.19 at 14:30, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 24/05/2019 09:39, Jan Beulich wrote:
>>>>> On 24.05.19 at 10:34, <jgross@xxxxxxxx> wrote:
>>> On 24/05/2019 08:38, Jan Beulich wrote:
>>>>>>> On 24.05.19 at 07:41, <jgross@xxxxxxxx> wrote:
>>>>> On 22/05/2019 12:10, Jan Beulich wrote:
>>>>>>>>> On 22.05.19 at 11:45, <jgross@xxxxxxxx> wrote:
>>>>>>> --- a/xen/arch/x86/hvm/hvm.c
>>>>>>> +++ b/xen/arch/x86/hvm/hvm.c
>>>>>>> @@ -3185,22 +3185,6 @@ static enum hvm_translation_result __hvm_copy(
>>>>>>>  
>>>>>>>      ASSERT(is_hvm_vcpu(v));
>>>>>>>  
>>>>>>> -    /*
>>>>>>> -     * XXX Disable for 4.1.0: PV-on-HVM drivers will do grant-table ops
>>>>>>> -     * such as query_size. Grant-table code currently does 
> copy_to/from_guest
>>>>>>> -     * accesses under the big per-domain lock, which this test would 
> disallow.
>>>>>>> -     * The test is not needed until we implement sleeping-on-waitqueue 
>>>>>>> when
>>>>>>> -     * we access a paged-out frame, and that's post 4.1.0 now.
>>>>>>> -     */
>>>>>>> -#if 0
>>>>>>> -    /*
>>>>>>> -     * If the required guest memory is paged out, this function may 
>>>>>>> sleep.
>>>>>>> -     * Hence we bail immediately if called from atomic context.
>>>>>>> -     */
>>>>>>> -    if ( in_atomic() )
>>>>>>> -        return HVMTRANS_unhandleable;
>>>>>>> -#endif
>>>>>> Dealing with this TODO item is of course much appreciated, but
>>>>>> should it really be deleted altogether? The big-domain-lock issue
>>>>>> is gone afair, in which case dropping the #if 0 would seem
>>>>>> possible to me, even if it's not strictly needed without the sleep-
>>>>>> on-waitqueue behavior mentioned.
>>>>> I just had a look and found the following path:
>>>>>
>>>>> do_domctl() (takes domctl_lock and hypercall_deadlock_mutex)
>>>>>   arch_do_domctl()
>>>>>     raw_copy_from_guest()
>>>>>       copy_from_user_hvm()
>>>>>         hvm_copy_from_guest_linear()
>>>>>           __hvm_copy()
>>>>>
>>>>> So no, we can't do the in_atomic() test IMO.
>>>> Oh, right - that's a PVH constraint that could probably not even
>>>> be thought of that the time the comment was written. I'm still
>>>> of the opinion though that at least the still applicable part of
>>>> the comment should be kept in place. Whether this means also
>>>> keeping in_atomic() itself is then an independent question, i.e.
>>>> I wouldn't consider it overly bad if there was no implementation
>>>> in the tree, but the above still served as documentation of what
>>>> would need to be re-added. Still my preference would be for it
>>>> to be kept.
>>> Would you be okay with replacing the removed stuff above with:
>>>
>>> /*
>>>  * If the required guest memory is paged out this function may sleep.
>>>  * So in theory we should bail out if called in atomic context.
>>>  * Unfortunately this is true for PVH dom0 doing domctl calls which
>> ... this is true at least for ...
>>
>>>  * holds the domctl lock when accessing dom0 memory. OTOH dom0 memory
>>>  * should never be paged out, so we are fine without testing for
>>>  * atomic context.
>>>  */
>> Not sure about this Dom0-specific remark: Are we certain there are
>> no other paths, similar to the gnttab one having been mentioned till
>> now?
> 
> Why is __hvm_copy() so special?  It is just one of many places which can
> end up touching guest memory.

Are you sure? I think everything that can touch guest (HVM) memory
actually ends up calling into this function.

Jan



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