[Xen-devel] [RFC 08/16] xen/arm: p2m: Handle translation fault in get_page_from_gva

[Julien Grall <julien.grall@xxxxxxx>]

A follow-up patch will re-purpose the valid bit of LPAE entries to
generate fault even on entry containing valid information.

This means that when translation a guest VA to guest PA (e.g IPA) will
fail if the Stage-2 entries used have the valid bit unset. Because of
that, we need to fallback to walk the page-table in software to check
whether the fault was expected.

This patch adds the software page-table walk on all the translation
fault. It would be possible in the future to avoid pointless walk when
the fault in PAR_EL1 is not a translation fault.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---

There are a couple of TODO in the code. They are clean-up and performance
improvement (e.g when the fault cannot be handled) that could be delayed after
the series has been merged.
---
 xen/arch/arm/p2m.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 58 insertions(+), 7 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 2a1e7e9be2..ec956bc151 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -13,6 +13,7 @@
 #include <public/vm_event.h>
 #include <asm/flushtlb.h>
 #include <asm/event.h>
+#include <asm/guest_walk.h>
 #include <asm/hardirq.h>
 #include <asm/page.h>
 
@@ -1438,6 +1439,8 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
vaddr_t va,
     struct page_info *page = NULL;
     paddr_t maddr = 0;
     uint64_t par;
+    mfn_t mfn;
+    p2m_type_t t;
 
     /*
      * XXX: To support a different vCPU, we would need to load the
@@ -1454,8 +1457,30 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
vaddr_t va,
     par = gvirt_to_maddr(va, &maddr, flags);
     p2m_read_unlock(p2m);
 
+    /*
+     * gvirt_to_maddr may fail if the entry does not have the valid bit
+     * set. Fallback
+     * to the second method:
+     *  1) Translate the VA to IPA using software lookup -> Stage-1 page-table
+     *  may not be accessible because the stage-2 entries may have valid
+     *  bit unset.
+     *  2) Software lookup of the MFN
+     *
+     * Note that when memaccess is enabled, we instead all directly
+     * p2m_mem_access_check_and_get_page(...). Because the function is a
+     * a variant of the methods described above, it will be able to
+     * handle entry with valid bit unset.
+     *
+     * TODO: Integrate more nicely memaccess with the rest of the
+     * function.
+     * TODO: Use the fault error in PAR_EL1 to avoid pointless
+     *  translation.
+     */
     if ( par )
     {
+        paddr_t ipa;
+        unsigned int perms;
+
         /*
          * When memaccess is enabled, the translation GVA to MADDR may
          * have failed because of a permission fault.
@@ -1463,20 +1488,46 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
vaddr_t va,
         if ( p2m->mem_access_enabled )
             return p2m_mem_access_check_and_get_page(va, flags, v);
 
-        dprintk(XENLOG_G_DEBUG,
-                "%pv: gvirt_to_maddr failed va=%#"PRIvaddr" flags=0x%lx 
par=%#"PRIx64"\n",
-                v, va, flags, par);
-        return NULL;
+        /*
+         * The software stage-1 table walk can still fail, e.g, if the
+         * GVA is not mapped.
+         */
+        if ( !guest_walk_tables(v, va, &ipa, &perms) )
+        {
+            dprintk(XENLOG_G_DEBUG, "%pv: Failed to walk page-table va 
%#"PRIvaddr"\n", v, va);
+            return NULL;
+        }
+
+        /*
+         * Check permission that are assumed by the caller. For instance
+         * in case of guestcopy, the caller assumes that the translated
+         * page can be accessed with the requested permissions. If this
+         * is not the case, we should fail.
+         *
+         * Please note that we do not check for the GV2M_EXEC
+         * permission. This is fine because the hardware-based translation
+         * instruction does not test for execute permissions.
+         */
+        if ( (flags & GV2M_WRITE) && !(perms & GV2M_WRITE) )
+            return NULL;
+
+        mfn = p2m_lookup(d, gaddr_to_gfn(ipa), &t);
+        if ( mfn_eq(INVALID_MFN, mfn) )
+            return NULL;
+
+        /* We consider that RAM is always mapped read-write */
     }
+    else
+        mfn = maddr_to_mfn(maddr);
 
-    if ( !mfn_valid(maddr_to_mfn(maddr)) )
+    if ( !mfn_valid(mfn) )
     {
         dprintk(XENLOG_G_DEBUG, "%pv: Invalid MFN %#"PRI_mfn"\n",
-                v, mfn_x(maddr_to_mfn(maddr)));
+                v, mfn_x(mfn));
         return NULL;
     }
 
-    page = mfn_to_page(maddr_to_mfn(maddr));
+    page = mfn_to_page(mfn);
     ASSERT(page);
 
     if ( unlikely(!get_page(page, d)) )
-- 
2.11.0


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