[PATCH v2 1/3] x86/mem_sharing: option to enforce fork starting with empty p2m

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

Add option to the fork memop to enforce a start with an empty p2m.
Pre-populating special pages to the fork tend to be necessary only when setting
up forks to be fully functional with a toolstack or if the fork makes use of
them in some way. For short-lived forks these pages are optional and starting
with an empty p2m has advantages both in terms of reset performance as well as
easier reasoning about the state of the fork after creation.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
---
v2: rename flag to empty_p2m, add assert at the end and move
     vAPIC page mapping skipping logic into where its mapped
---
 tools/include/xenctrl.h               |  3 ++-
 tools/libs/ctrl/xc_memshr.c           |  5 +++-
 xen/arch/x86/hvm/vmx/vmx.c            |  5 ++++
 xen/arch/x86/include/asm/hvm/domain.h |  4 +++-
 xen/arch/x86/mm/mem_sharing.c         | 33 +++++++++++++++++----------
 xen/include/public/memory.h           |  4 ++--
 6 files changed, 37 insertions(+), 17 deletions(-)

diff --git a/tools/include/xenctrl.h b/tools/include/xenctrl.h
index 95bd5eca67..26766ec19f 100644
--- a/tools/include/xenctrl.h
+++ b/tools/include/xenctrl.h
@@ -2281,7 +2281,8 @@ int xc_memshr_fork(xc_interface *xch,
                    uint32_t source_domain,
                    uint32_t client_domain,
                    bool allow_with_iommu,
-                   bool block_interrupts);
+                   bool block_interrupts,
+                   bool empty_p2m);
 
 /*
  * Note: this function is only intended to be used on short-lived forks that
diff --git a/tools/libs/ctrl/xc_memshr.c b/tools/libs/ctrl/xc_memshr.c
index a6cfd7dccf..0143f9ddea 100644
--- a/tools/libs/ctrl/xc_memshr.c
+++ b/tools/libs/ctrl/xc_memshr.c
@@ -240,7 +240,8 @@ int xc_memshr_debug_gref(xc_interface *xch,
 }
 
 int xc_memshr_fork(xc_interface *xch, uint32_t pdomid, uint32_t domid,
-                   bool allow_with_iommu, bool block_interrupts)
+                   bool allow_with_iommu, bool block_interrupts,
+                   bool empty_p2m)
 {
     xen_mem_sharing_op_t mso;
 
@@ -253,6 +254,8 @@ int xc_memshr_fork(xc_interface *xch, uint32_t pdomid, 
uint32_t domid,
         mso.u.fork.flags |= XENMEM_FORK_WITH_IOMMU_ALLOWED;
     if ( block_interrupts )
         mso.u.fork.flags |= XENMEM_FORK_BLOCK_INTERRUPTS;
+    if ( empty_p2m )
+        mso.u.fork.flags |= XENMEM_FORK_EMPTY_P2M;
 
     return xc_memshr_memop(xch, domid, &mso);
 }
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index c075370f64..5e60c92d5c 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -424,6 +424,11 @@ static void cf_check domain_creation_finished(struct 
domain *d)
     if ( !has_vlapic(d) || mfn_eq(apic_access_mfn, INVALID_MFN) )
         return;
 
+#ifdef CONFIG_MEM_SHARING
+    if ( d->arch.hvm.mem_sharing.empty_p2m )
+        return;
+#endif
+
     ASSERT(epte_get_entry_emt(d, gfn, apic_access_mfn, 0, &ipat,
                               p2m_mmio_direct) == MTRR_TYPE_WRBACK);
     ASSERT(ipat);
diff --git a/xen/arch/x86/include/asm/hvm/domain.h 
b/xen/arch/x86/include/asm/hvm/domain.h
index 698455444e..22a17c36c5 100644
--- a/xen/arch/x86/include/asm/hvm/domain.h
+++ b/xen/arch/x86/include/asm/hvm/domain.h
@@ -31,7 +31,9 @@
 #ifdef CONFIG_MEM_SHARING
 struct mem_sharing_domain
 {
-    bool enabled, block_interrupts;
+    bool enabled;
+    bool block_interrupts;
+    bool empty_p2m;
 
     /*
      * When releasing shared gfn's in a preemptible manner, recall where
diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 15e6a7ed81..ef67285a98 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -1643,7 +1643,8 @@ static int bring_up_vcpus(struct domain *cd, struct 
domain *d)
     return 0;
 }
 
-static int copy_vcpu_settings(struct domain *cd, const struct domain *d)
+static int copy_vcpu_settings(struct domain *cd, const struct domain *d,
+                              bool empty_p2m)
 {
     unsigned int i;
     struct p2m_domain *p2m = p2m_get_hostp2m(cd);
@@ -1660,7 +1661,7 @@ static int copy_vcpu_settings(struct domain *cd, const 
struct domain *d)
 
         /* Copy & map in the vcpu_info page if the guest uses one */
         vcpu_info_mfn = d_vcpu->vcpu_info_mfn;
-        if ( !mfn_eq(vcpu_info_mfn, INVALID_MFN) )
+        if ( !empty_p2m && !mfn_eq(vcpu_info_mfn, INVALID_MFN) )
         {
             mfn_t new_vcpu_info_mfn = cd_vcpu->vcpu_info_mfn;
 
@@ -1807,17 +1808,18 @@ static int copy_special_pages(struct domain *cd, struct 
domain *d)
     return 0;
 }
 
-static int copy_settings(struct domain *cd, struct domain *d)
+static int copy_settings(struct domain *cd, struct domain *d,
+                         bool empty_p2m)
 {
     int rc;
 
-    if ( (rc = copy_vcpu_settings(cd, d)) )
+    if ( (rc = copy_vcpu_settings(cd, d, empty_p2m)) )
         return rc;
 
     if ( (rc = hvm_copy_context_and_params(cd, d)) )
         return rc;
 
-    if ( (rc = copy_special_pages(cd, d)) )
+    if ( !empty_p2m && (rc = copy_special_pages(cd, d)) )
         return rc;
 
     copy_tsc(cd, d);
@@ -1826,9 +1828,11 @@ static int copy_settings(struct domain *cd, struct 
domain *d)
     return rc;
 }
 
-static int fork(struct domain *cd, struct domain *d)
+static int fork(struct domain *cd, struct domain *d, uint16_t flags)
 {
     int rc = -EBUSY;
+    bool block_interrupts = flags & XENMEM_FORK_BLOCK_INTERRUPTS;
+    bool empty_p2m = flags & XENMEM_FORK_EMPTY_P2M;
 
     if ( !cd->controller_pause_count )
         return rc;
@@ -1856,7 +1860,13 @@ static int fork(struct domain *cd, struct domain *d)
     if ( (rc = bring_up_vcpus(cd, d)) )
         goto done;
 
-    rc = copy_settings(cd, d);
+    if ( !(rc = copy_settings(cd, d, empty_p2m)) )
+    {
+        cd->arch.hvm.mem_sharing.block_interrupts = block_interrupts;
+
+        if ( (cd->arch.hvm.mem_sharing.empty_p2m = empty_p2m) )
+            ASSERT(page_list_empty(&cd->page_list));
+    }
 
  done:
     if ( rc && rc != -ERESTART )
@@ -1920,7 +1930,7 @@ static int mem_sharing_fork_reset(struct domain *d)
     }
     spin_unlock_recursive(&d->page_alloc_lock);
 
-    rc = copy_settings(d, pd);
+    rc = copy_settings(d, pd, d->arch.hvm.mem_sharing.empty_p2m);
 
     domain_unpause(d);
 
@@ -2190,7 +2200,8 @@ int 
mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
         if ( mso.u.fork.pad )
             goto out;
         if ( mso.u.fork.flags &
-             ~(XENMEM_FORK_WITH_IOMMU_ALLOWED | XENMEM_FORK_BLOCK_INTERRUPTS) )
+             ~(XENMEM_FORK_WITH_IOMMU_ALLOWED | XENMEM_FORK_BLOCK_INTERRUPTS |
+               XENMEM_FORK_EMPTY_P2M) )
             goto out;
 
         rc = rcu_lock_live_remote_domain_by_id(mso.u.fork.parent_domain,
@@ -2212,14 +2223,12 @@ int 
mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
             goto out;
         }
 
-        rc = fork(d, pd);
+        rc = fork(d, pd, mso.u.fork.flags);
 
         if ( rc == -ERESTART )
             rc = hypercall_create_continuation(__HYPERVISOR_memory_op,
                                                "lh", XENMEM_sharing_op,
                                                arg);
-        else if ( !rc && (mso.u.fork.flags & XENMEM_FORK_BLOCK_INTERRUPTS) )
-            d->arch.hvm.mem_sharing.block_interrupts = true;
 
         rcu_unlock_domain(pd);
         break;
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index a1a0f0233a..d44c256b3c 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -543,10 +543,10 @@ struct xen_mem_sharing_op {
         } debug;
         struct mem_sharing_op_fork {      /* OP_FORK */
             domid_t parent_domain;        /* IN: parent's domain id */
-/* Only makes sense for short-lived forks */
+/* These flags only makes sense for short-lived forks */
 #define XENMEM_FORK_WITH_IOMMU_ALLOWED (1u << 0)
-/* Only makes sense for short-lived forks */
 #define XENMEM_FORK_BLOCK_INTERRUPTS   (1u << 1)
+#define XENMEM_FORK_EMPTY_P2M          (1u << 2)
             uint16_t flags;               /* IN: optional settings */
             uint32_t pad;                 /* Must be set to 0 */
         } fork;
-- 
2.25.1