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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----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.

  Paul

> Jan


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