Re: [Qemu-devel] [PATCH QEMU-XEN v6 4/8] xen: Switch uses of xc_map_foreign_range into xc_map_foreign_pages

[Stefano Stabellini]

On Wed, 9 Dec 2015, Ian Campbell wrote:
> On Thu, 2015-12-03 at 11:23 +0000, Ian Campbell wrote:
> > diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> > index 5e324ef..c96d974 100644
> > --- a/hw/display/xenfb.c
> > +++ b/hw/display/xenfb.c
> > @@ -104,9 +104,8 @@ static int common_bind(struct common *c)
> >      if (xenstore_read_fe_int(&c->xendev, "event-channel", 
> > &c->xendev.remote_port) == -1)
> >     return -1;
> >  
> > -    c->page = xc_map_foreign_range(xen_xc, c->xendev.dom,
> > -                              XC_PAGE_SIZE,
> > -                              PROT_READ | PROT_WRITE, mfn);
> > +    c->page = xc_map_foreign_pages(xen_xc, c->xendev.dom,
> > +                                   PROT_READ | PROT_WRITE, &mfn, 1);
> 
> This doesn't build for i386 userspace, since mfn is a uint64_t but
> xc_map_foreign_pages() wants a xen_pfn_t * (where xen_pfn_t == unsigned
> long on x86).
> 
> Until now that was just a truncation which was already checked for with:
> 
>     uint64_t mfn;
> 
>     if (xenstore_read_fe_uint64(&c->xendev, "page-ref", &mfn) == -1)
>         return -1;
>     assert(mfn == (xen_pfn_t)mfn);
> 
> I think in principal passing "(xen_pfn_t *)&mfn" would ok (since it is a
> singleton array in this case), but I was thinking of going a bit further
> and:
> 
> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> index 8b86b4a..54585fa 100644
> --- a/hw/display/xenfb.c
> +++ b/hw/display/xenfb.c
> @@ -95,11 +95,13 @@ struct XenFB {
>  
>  static int common_bind(struct common *c)
>  {
> -    uint64_t mfn;
> +    uint64_t val;
> +    xen_pfn_t mfn;
>  
> -    if (xenstore_read_fe_uint64(&c->xendev, "page-ref", &mfn) == -1)
> +    if (xenstore_read_fe_uint64(&c->xendev, "page-ref", &val) == -1)
>       return -1;
> -    assert(mfn == (xen_pfn_t)mfn);
> +    mfn = (xen_pfn_t)val;
> +    assert(val == mfn);
>  
>      if (xenstore_read_fe_int(&c->xendev, "event-channel", 
> &c->xendev.remote_port) == -1)
>       return -1;
> 
> Stefano, what do you think/prefer? An alternative to the above

I like this change because it makes the code more obvious