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

[Ian Campbell <ian.campbell@citrix.com>]

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've not spotted any other similar constructs, xenfb is a bit unusual here
in that it apparently uses foreign mappings rather than grants like most
drivers do.

Ian.