[Xen-devel] [PATCH v2] libxc: introduce a per architecture scratch pfn for temporary grant mapping

[Julien Grall <julien.grall@xxxxxxxxxx>]

The code to initialize the grant table in libxc uses
xc_domain_maximum_gpfn() + 1 to get a guest pfn for mapping the grant
frame and to initialize it.

This solution has two major issues:
    - The check of the return of xc_domain_maximum_gpfn is buggy because
    xen_pfn_t is unsigned and in case of an error -ERRNO is returned.
    Which is never catch with ( pfn <= 0 ).
    - The guest memory layout maybe filled up to the end, i.e
    xc_domain_maximum_gpfn() + 1 gives either 0 or an invalid PFN due to
    hardware limitation.

Futhermore, on ARM, xc_domain_maximum_gpfn() is not implemented and
return -ENOSYS. This will make libxc to use always the same PFN which
may colapse with an already mapped region (see xen/include/public/arch-arm.h
for the layout).

This patch only address the problem for ARM, the x86 version use the same
behavior (ie xc_domain_maximum_gpfn() + 1), as I'm not familiar with Xen x86.

A new function xc_core_arch_get_scratch_gpfn is introduced to be able to
choose the gpfn per architecture.

For the ARM version, we use the GUEST_GNTTAB_GUEST which is the base of
the region by the guest to map the grant table. At the build time,
nothing is mapped there.

At the same time correctly check the return of xc_domain_maximum_gpfn
for x86.

Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>

---
    This patch is candidate for backport to Xen 4.5. With the support of
    MMIO passthrough, we may end up to erase an MMIO region.

    I don't think it's required for Xen 4.4.

    I chose to take this appproach after the discussion on implementing
    XENMEM_maximum_gpfn on ARM (https://patches.linaro.org/32894/).

    This patch has only been built tested on x86 and the same behavior
    has been kept (i.e xc_domain_maximum_gpfn() + 1). I would be happy
    if someone for x86 world is looking for a possible solution.

    Changes in v2:
        - x86: The +1 was missing to get the scratch pfn
        - Fix mispelling in comment in xc_core_arch_get_gpfn for ARM
---
 tools/libxc/xc_core.h     |  3 +++
 tools/libxc/xc_core_arm.c | 17 +++++++++++++++++
 tools/libxc/xc_core_x86.c | 17 +++++++++++++++++
 tools/libxc/xc_dom_boot.c | 18 ++++++++++++------
 4 files changed, 49 insertions(+), 6 deletions(-)

diff --git a/tools/libxc/xc_core.h b/tools/libxc/xc_core.h
index 10cbfca..5867030 100644
--- a/tools/libxc/xc_core.h
+++ b/tools/libxc/xc_core.h
@@ -148,6 +148,9 @@ int xc_core_arch_map_p2m_writable(xc_interface *xch, 
unsigned int guest_width,
                                   shared_info_any_t *live_shinfo,
                                   xen_pfn_t **live_p2m, unsigned long *pfnp);
 
+int xc_core_arch_get_scratch_gpfn(xc_interface *xch, domid_t domid,
+                                  xen_pfn_t *gpfn);
+
 
 #if defined (__i386__) || defined (__x86_64__)
 # include "xc_core_x86.h"
diff --git a/tools/libxc/xc_core_arm.c b/tools/libxc/xc_core_arm.c
index 2fbcf3f..16508e7 100644
--- a/tools/libxc/xc_core_arm.c
+++ b/tools/libxc/xc_core_arm.c
@@ -96,6 +96,23 @@ xc_core_arch_map_p2m_writable(xc_interface *xch, unsigned 
int guest_width, xc_do
     return xc_core_arch_map_p2m_rw(xch, dinfo, info,
                                    live_shinfo, live_p2m, pfnp, 1);
 }
+
+int
+xc_core_arch_get_scratch_gpfn(xc_interface *xch, domid_t domid,
+                              xen_pfn_t *gpfn)
+{
+    /*
+     * The Grant Table region space is not used until the guest is
+     * booting. Use the first page for the scratch pfn.
+     */
+    XC_BUILD_BUG_ON(GUEST_GNTTAB_SIZE < XC_PAGE_SIZE);
+
+    *gpfn = GUEST_GNTTAB_BASE >> XC_PAGE_SHIFT;
+
+    return 0;
+}
+
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/libxc/xc_core_x86.c b/tools/libxc/xc_core_x86.c
index f05060a..fac99ec 100644
--- a/tools/libxc/xc_core_x86.c
+++ b/tools/libxc/xc_core_x86.c
@@ -205,6 +205,23 @@ xc_core_arch_map_p2m_writable(xc_interface *xch, unsigned 
int guest_width, xc_do
     return xc_core_arch_map_p2m_rw(xch, dinfo, info,
                                    live_shinfo, live_p2m, pfnp, 1);
 }
+
+int
+xc_core_arch_get_scratch_gpfn(xc_interface *xch, domid_t domid,
+                              xen_pfn_t *gpfn)
+{
+    int rc;
+
+    rc = xc_domain_maximum_gpfn(xch, domid);
+
+    if ( rc <= 0 )
+        return rc;
+
+    *gpfn = rc + 1;
+
+    return 0;
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/libxc/xc_dom_boot.c b/tools/libxc/xc_dom_boot.c
index f0a1c64..a141eb5 100644
--- a/tools/libxc/xc_dom_boot.c
+++ b/tools/libxc/xc_dom_boot.c
@@ -33,6 +33,7 @@
 
 #include "xg_private.h"
 #include "xc_dom.h"
+#include "xc_core.h"
 #include <xen/hvm/params.h>
 #include <xen/grant_table.h>
 
@@ -365,7 +366,7 @@ int xc_dom_gnttab_hvm_seed(xc_interface *xch, domid_t domid,
                            domid_t xenstore_domid)
 {
     int rc;
-    xen_pfn_t max_gfn;
+    xen_pfn_t scratch_gpfn;
     struct xen_add_to_physmap xatp = {
         .domid = domid,
         .space = XENMAPSPACE_grant_table,
@@ -375,16 +376,21 @@ int xc_dom_gnttab_hvm_seed(xc_interface *xch, domid_t 
domid,
         .domid = domid,
     };
 
-    max_gfn = xc_domain_maximum_gpfn(xch, domid);
-    if ( max_gfn <= 0 ) {
+    rc = xc_core_arch_get_scratch_gpfn(xch, domid, &scratch_gpfn);
+    if ( rc < 0 )
+    {
         xc_dom_panic(xch, XC_INTERNAL_ERROR,
-                     "%s: failed to get max gfn "
+                     "%s: failed to get a scratch gfn "
                      "[errno=%d]\n",
                      __FUNCTION__, errno);
         return -1;
     }
-    xatp.gpfn = max_gfn + 1;
-    xrfp.gpfn = max_gfn + 1;
+    xatp.gpfn = scratch_gpfn;
+    xrfp.gpfn = scratch_gpfn;
+
+    xc_dom_printf(xch, "%s: called, pfn=0x%"PRI_xen_pfn, __FUNCTION__,
+                  scratch_gpfn);
+
 
     rc = do_memory_op(xch, XENMEM_add_to_physmap, &xatp, sizeof(xatp));
     if ( rc != 0 )
-- 
2.1.4


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