Re: [Xen-devel] [PATCH] x86/hvm/viridian: zero and check vcpu context __pad field

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.03.16 at 17:16, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 30 March 2016 15:22
>> To: Paul Durrant
>> Cc: Andrew Cooper; xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
>> Subject: RE: [PATCH] x86/hvm/viridian: zero and check vcpu context __pad
>> field
>> 
>> >>> On 30.03.16 at 15:19, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >>  -----Original Message-----
>> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: 30 March 2016 14:17
>> >> To: Paul Durrant
>> >> Cc: Andrew Cooper; xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
>> >> Subject: RE: [PATCH] x86/hvm/viridian: zero and check vcpu context
>> __pad
>> >> field
>> >>
>> >> >>> On 30.03.16 at 13:26, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> >> Sent: 30 March 2016 12:23
>> >> >> >>> On 30.03.16 at 12:32, <paul.durrant@xxxxxxxxxx> wrote:
>> >> >> > --- a/xen/arch/x86/hvm/viridian.c
>> >> >> > +++ b/xen/arch/x86/hvm/viridian.c
>> >> >> > @@ -824,6 +824,8 @@ static int viridian_save_vcpu_ctxt(struct
>> domain
>> >> *d,
>> >> >> hvm_domain_context_t *h)
>> >> >> >      for_each_vcpu( d, v ) {
>> >> >> >          struct hvm_viridian_vcpu_context ctxt;
>> >> >> >
>> >> >> > +        memset(&ctxt, 0, sizeof(ctxt));
>> >> >>
>> >> >> How about just adding an empty initializer to the declaration?
>> >> >>
>> >> >
>> >> > I think having a 'zero the entire struct' call at the start is better 
>> >> > as 
> it
>> >> > will cover any additions made to the struct in future. It's what I had
>> >> > mistakenly assumed was already there. In fact I think adding a similar 
> call
>> >> > into the domain context save function would probably be worthwhile.
>> >>
>> >> And how does the initializer approach not fulfill that intention?
>> >>
>> >
>> > Because any time anyone adds another field they have to remember to
>> add
>> > another initializer, which is what I forgot to do. This approach OTOH is
>> > failsafe.
>> 
>> But note how I said "an empty initializer": When there is an
>> initializer at all, all fields not mentioned in the initializer will get
>> default initialized (i.e. zeroed). Hence an empty initializer
>> clears the entire structure.
>> 
> 
> Ah, you mean C99 initializer style. That would be neater.

Not really - "static struct s s = {};" was allowed in C89 too iirc.

Jan


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