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

[Juergen Gross <jgross@xxxxxxxx>]

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
 * 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.
 */


Juergen

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