[Xen-devel] [PATCH v3 17/18] x86/mem_sharing: reset a fork

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>]

Implement hypercall that allows a fork to shed all memory that got allocated
for it during its execution and re-load its vCPU context from the parent VM.
This allows the forked VM to reset into the same state the parent VM is in a
faster way then creating a new fork would be. Measurements show about a 2x
speedup during normal fuzzing operations. Performance may vary depending how
much memory got allocated for the forked VM. If it has been completely
deduplicated from the parent VM then creating a new fork would likely be more
performant.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
---
v3: use page_list_for_each_safe so we don't have to loop twice
    add comments explaining the lack of hypercall continuation
    only remove pages doing reset that are of sharable type
---
 xen/arch/x86/mm/mem_sharing.c | 80 +++++++++++++++++++++++++++++++++++
 xen/include/public/memory.h   |  1 +
 2 files changed, 81 insertions(+)

diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index d544801681..f16570b01b 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -1607,6 +1607,63 @@ static int mem_sharing_fork(struct domain *d, struct 
domain *cd)
     return 0;
 }
 
+/*
+ * The fork reset operation is intended to be used on short-lived forks only.
+ * There is no hypercall continuation operation implemented for this reason.
+ * For forks that obtain a larger memory footprint it is likely going to be
+ * more performant to create a new fork instead of resetting an existing one.
+ *
+ * TODO: In case this hypercall would become useful on forks with larger memory
+ * footprints the hypercall continuation should be implemented.
+ */
+static int mem_sharing_fork_reset(struct domain *d, struct domain *cd)
+{
+    int rc;
+    struct p2m_domain* p2m = p2m_get_hostp2m(cd);
+    struct page_info *page, *tmp;
+
+    if ( !d->controller_pause_count &&
+         (rc = domain_pause_by_systemcontroller(d)) )
+        return rc;
+
+    page_list_for_each_safe(page, tmp, &cd->page_list)
+    {
+        p2m_type_t p2mt;
+        p2m_access_t p2ma;
+        gfn_t gfn;
+        mfn_t mfn = page_to_mfn(page);
+
+        if ( !mfn_valid(mfn) )
+            continue;
+
+        gfn = mfn_to_gfn(cd, mfn);
+        mfn = __get_gfn_type_access(p2m, gfn_x(gfn), &p2mt, &p2ma,
+                                    0, NULL, false);
+
+        /* only remove pages that can be re-populated using sharing */
+        if ( !p2m_is_sharable(p2mt) )
+            continue;
+
+        /* take an extra reference */
+        if ( !get_page(page, cd) )
+            continue;
+
+        rc = p2m->set_entry(p2m, gfn, INVALID_MFN, PAGE_ORDER_4K,
+                            p2m_invalid, p2m_access_rwx, -1);
+        ASSERT(!rc);
+
+        put_page_alloc_ref(page);
+        put_page(page);
+    }
+
+    if ( (rc = hvm_copy_context_and_params(d, cd)) )
+        return rc;
+
+    fork_tsc(d, cd);
+
+    return 0;
+}
+
 int mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
 {
     int rc;
@@ -1909,6 +1966,29 @@ int 
mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
         break;
     }
 
+    case XENMEM_sharing_op_fork_reset:
+    {
+        struct domain *pd;
+
+        rc = -EINVAL;
+        if ( mso.u.fork._pad[0] || mso.u.fork._pad[1] ||
+             mso.u.fork._pad[2] )
+            goto out;
+
+        rc = -ENOSYS;
+        if ( !d->parent )
+            goto out;
+
+        rc = rcu_lock_live_remote_domain_by_id(d->parent->domain_id, &pd);
+        if ( rc )
+            goto out;
+
+        rc = mem_sharing_fork_reset(pd, d);
+
+        rcu_unlock_domain(pd);
+        break;
+    }
+
     default:
         rc = -ENOSYS;
         break;
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index 90a3f4498e..e3d063e22e 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -483,6 +483,7 @@ DEFINE_XEN_GUEST_HANDLE(xen_mem_access_op_t);
 #define XENMEM_sharing_op_audit             7
 #define XENMEM_sharing_op_range_share       8
 #define XENMEM_sharing_op_fork              9
+#define XENMEM_sharing_op_fork_reset        10
 
 #define XENMEM_SHARING_OP_S_HANDLE_INVALID  (-10)
 #define XENMEM_SHARING_OP_C_HANDLE_INVALID  (-9)
-- 
2.20.1


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