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

[Juergen Gross <jgross@suse.com>]

On 22/05/2019 12:10, Jan Beulich wrote:
>>>> On 22.05.19 at 11:45, <jgross@suse.com> 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.


Juergen

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