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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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?

Jan



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