Re: [Minios-devel] [UNIKRAFT PATCH 6/6] plat/xen: Add grant table support for Xen (x86_64)

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jun 27, 2018 at 03:49:21PM +0200, Yuri Volchkov wrote:
> > +grant_ref_t gnttab_grant_access(domid_t domid, unsigned long pfn,
> > +           int readonly)
> Is it supposed to be a mfn, not pfn?
> 
> > +{
> > +   grant_ref_t gref;
> > +
> > +   gref = get_free_entry();
> > +   gnttab.table[gref].frame = pfn;
> > +   gnttab.table[gref].domid = domid;
> > +   wmb();
> Why the barrier is here not after setting the readonly flag? MiniOS and
> Linux are doing the same, so I assume that is the right way. Maybe you
> could enlighten me? :)
> 

Because the purpose is to make sure the write to the flags field happens
*after* the write to frame and domid fields. I.e. the change of the flag
will be visible only after the changes to the other fields are visible.

There are probably some paring rmb() in the code, but I haven't
looked...

Wei.

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