[PATCH v5 07/10] domain: map/unmap GADDR based shared guest areas

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

From: Jan Beulich <jbeulich@xxxxxxxx>

The registration by virtual/linear address has downsides: At least on
x86 the access is expensive for HVM/PVH domains. Furthermore for 64-bit
PV domains the areas are inaccessible (and hence cannot be updated by
Xen) when in guest-user mode, and for HVM guests they may be
inaccessible when Meltdown mitigations are in place. (There are yet
more issues.)

In preparation of the introduction of new vCPU operations allowing to
register the respective areas (one of the two is x86-specific) by
guest-physical address, flesh out the map/unmap functions.

Noteworthy differences from map_vcpu_info():
- areas can be registered more than once (and de-registered),
- remote vCPU-s are paused rather than checked for being down (which in
  principle can change right after the check),
- the domain lock is taken for a much smaller region.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/common/domain.c | 92 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 91 insertions(+), 1 deletion(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 47fc90271901..747bf5c87a8d 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1605,7 +1605,82 @@ int map_guest_area(struct vcpu *v, paddr_t gaddr, 
unsigned int size,
                    struct guest_area *area,
                    void (*populate)(void *dst, struct vcpu *v))
 {
-    return -EOPNOTSUPP;
+    struct domain *d = v->domain;
+    void *map = NULL;
+    struct page_info *pg = NULL;
+    int rc = 0;
+
+    if ( ~gaddr ) /* Map (i.e. not just unmap)? */
+    {
+        unsigned long gfn = PFN_DOWN(gaddr);
+        unsigned int align;
+        p2m_type_t p2mt;
+
+        if ( gfn != PFN_DOWN(gaddr + size - 1) )
+            return -ENXIO;
+
+#ifdef CONFIG_COMPAT
+        if ( has_32bit_shinfo(d) )
+            align = alignof(compat_ulong_t);
+        else
+#endif
+            align = alignof(xen_ulong_t);
+        if ( !IS_ALIGNED(gaddr, align) )
+            return -ENXIO;
+
+        rc = check_get_page_from_gfn(d, _gfn(gfn), false, &p2mt, &pg);
+        if ( rc )
+            return rc;
+
+        if ( !get_page_type(pg, PGT_writable_page) )
+        {
+            put_page(pg);
+            return -EACCES;
+        }
+
+        map = __map_domain_page_global(pg);
+        if ( !map )
+        {
+            put_page_and_type(pg);
+            return -ENOMEM;
+        }
+        map += PAGE_OFFSET(gaddr);
+    }
+
+    if ( v != current )
+    {
+        if ( !spin_trylock(&d->hypercall_deadlock_mutex) )
+        {
+            rc = -ERESTART;
+            goto unmap;
+        }
+
+        vcpu_pause(v);
+
+        spin_unlock(&d->hypercall_deadlock_mutex);
+    }
+
+    domain_lock(d);
+
+    if ( map && populate )
+        populate(map, v);
+
+    SWAP(area->pg, pg);
+    SWAP(area->map, map);
+
+    domain_unlock(d);
+
+    if ( v != current )
+        vcpu_unpause(v);
+
+ unmap:
+    if ( pg )
+    {
+        unmap_domain_page_global(map);
+        put_page_and_type(pg);
+    }
+
+    return rc;
 }
 
 /*
@@ -1616,9 +1691,24 @@ int map_guest_area(struct vcpu *v, paddr_t gaddr, 
unsigned int size,
 void unmap_guest_area(struct vcpu *v, struct guest_area *area)
 {
     struct domain *d = v->domain;
+    void *map;
+    struct page_info *pg;
 
     if ( v != current )
         ASSERT(atomic_read(&v->pause_count) | atomic_read(&d->pause_count));
+
+    domain_lock(d);
+    map = area->map;
+    area->map = NULL;
+    pg = area->pg;
+    area->pg = NULL;
+    domain_unlock(d);
+
+    if ( pg )
+    {
+        unmap_domain_page_global(map);
+        put_page_and_type(pg);
+    }
 }
 
 int default_initialise_vcpu(struct vcpu *v, XEN_GUEST_HANDLE_PARAM(void) arg)
-- 
2.42.0