Re: [Xen-devel] [PATCH v12 1/3] xen/mem_sharing: VM forking

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 26, 2020 at 08:02:22AM +0100, Jan Beulich wrote:
> On 25.03.2020 17:54, Roger Pau Monné wrote:
> > On Wed, Mar 25, 2020 at 04:42:07PM +0000, Julien Grall wrote:
> >> On 25/03/2020 16:34, Tamas K Lengyel wrote:
> >>>>> --- a/xen/common/domain.c
> >>>>> +++ b/xen/common/domain.c
> >>>>> @@ -1270,6 +1270,9 @@ int map_vcpu_info(struct vcpu *v, unsigned long 
> >>>>> gfn, unsigned offset)
> >>>>>
> >>>>>       v->vcpu_info = new_info;
> >>>>>       v->vcpu_info_mfn = page_to_mfn(page);
> >>>>> +#ifdef CONFIG_MEM_SHARING
> >>>>> +    v->vcpu_info_offset = offset;
> >>>>
> >>>> There's no need to introduce this field, you can just use v->vcpu_info
> >>>> & ~PAGE_MASK AFAICT.
> >>>
> >>> Just doing what you suggest above results in:
> >>>
> >>> mem_sharing.c:1603:55: error: invalid operands to binary & (have
> >>> ‘vcpu_info_t * const’ {aka ‘union <anonymous> * const’} and ‘long
> >>> int’)
> >>>                                       d_vcpu->vcpu_info & ~PAGE_MASK);
> >>>
> >>> I can of course cast the vcpu_info pointer to (long int), it's just a
> >>> bit ugly. Thoughts?
> >>
> >> FWIW, I will also need the offset for liveupdate. I have used (unsigned
> >> long)v->vcpu_info & ~PAGE_MASK so far but this is not really pretty.
> > 
> > I think using:
> > 
> > (vaddr_t)v->vcpu_info & ~PAGE_MASK
> > 
> > is acceptable, but that would require adding a vaddr_t typedef to x86.
> 
> I don't think vaddr_t is necessary given that all over the place we
> assume conversions between pointers and unsigned long to be fine.

Right, using unsigned long is indeed fine, but I also agree with Tamas
that it's slightly ugly and hence wanted to provide a 'cleaner'
suggestion.

Roger.