[Xen-devel] [PATCH 2/2] x86/HVM: fix preemption handling in do_hvm_op()

["Jan Beulich" <JBeulich@xxxxxxxx>]

Just like previously done for some mem-op hypercalls, undo preemption
using the interface structures (altering it in ways the caller may not
expect) and replace it by storing the continuation point in the high
bits of sub-operation argument.

This also changes the "nr" fields of struct xen_hvm_track_dirty_vram
(operation already limited to 1Gb worth of pages) and struct
xen_hvm_modified_memory to be only 32 bits wide, consistent with those
of struct xen_set_mem{type,access}. If that's not acceptable for some
reason, we'd need to shrink the HVMOP_op_bits (while still enforcing
the [then higher] limit resulting from the need to be able to encode
the continuation).

Whether (and if so how) to adjust xc_hvm_track_dirty_vram(),
xc_hvm_modified_memory(), xc_hvm_set_mem_type(), and
xc_hvm_set_mem_access() to reflect the 32-bit restriction on "nr" is
unclear: If the APIs need to remain stable, all four functions should
probably check that there was no truncation. Preferably their
parameters would be changed to uint32_t or unsigned int, though.

As a minor cleanup, along with introducing the switch-wide "pfn" the
redundant "d" is also being converted to a switch-wide one.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4050,20 +4050,25 @@ static int hvm_replace_event_channel(str
     return 0;
 }
 
+#define HVMOP_op_bits 32
+
 long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE_PARAM(void) arg)
 
 {
     struct domain *curr_d = current->domain;
+    unsigned long start_iter = op >> HVMOP_op_bits;
     long rc = 0;
 
-    switch ( op )
+    switch ( op &= ((1UL << HVMOP_op_bits) - 1) )
     {
+        struct domain *d;
+        unsigned long pfn;
+
     case HVMOP_set_param:
     case HVMOP_get_param:
     {
         struct xen_hvm_param a;
         struct hvm_ioreq_page *iorp;
-        struct domain *d;
         struct vcpu *v;
 
         if ( copy_from_guest(&a, arg, 1) )
@@ -4327,7 +4332,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_track_dirty_vram:
     {
         struct xen_hvm_track_dirty_vram a;
-        struct domain *d;
 
         if ( copy_from_guest(&a, arg, 1) )
             return -EFAULT;
@@ -4368,7 +4372,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_modified_memory:
     {
         struct xen_hvm_modified_memory a;
-        struct domain *d;
 
         if ( copy_from_guest(&a, arg, 1) )
             return -EFAULT;
@@ -4386,7 +4389,8 @@ long do_hvm_op(unsigned long op, XEN_GUE
             goto param_fail3;
 
         rc = -EINVAL;
-        if ( ((a.first_pfn + a.nr - 1) < a.first_pfn) ||
+        if ( a.nr < start_iter ||
+             ((a.first_pfn + a.nr - 1) < a.first_pfn) ||
              ((a.first_pfn + a.nr - 1) > domain_get_maximum_gpfn(d)) )
             goto param_fail3;
 
@@ -4394,9 +4398,8 @@ long do_hvm_op(unsigned long op, XEN_GUE
         if ( !paging_mode_log_dirty(d) )
             goto param_fail3;
 
-        while ( a.nr > 0 )
+        for ( pfn = a.first_pfn + start_iter; a.nr > start_iter; ++pfn )
         {
-            unsigned long pfn = a.first_pfn;
             struct page_info *page;
 
             page = get_page_from_gfn(d, pfn, NULL, P2M_UNSHARE);
@@ -4409,16 +4412,13 @@ long do_hvm_op(unsigned long op, XEN_GUE
                 put_page(page);
             }
 
-            a.first_pfn++;
-            a.nr--;
+            ++pfn;
+            ++start_iter;
 
             /* Check for continuation if it's not the last interation */
-            if ( a.nr > 0 && hypercall_preempt_check() )
+            if ( a.nr > start_iter && hypercall_preempt_check() )
             {
-                if ( __copy_to_guest(arg, &a, 1) )
-                    rc = -EFAULT;
-                else
-                    rc = -EAGAIN;
+                rc = -EAGAIN;
                 break;
             }
         }
@@ -4431,7 +4431,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_get_mem_type:
     {
         struct xen_hvm_get_mem_type a;
-        struct domain *d;
         p2m_type_t t;
 
         if ( copy_from_guest(&a, arg, 1) )
@@ -4475,7 +4474,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_set_mem_type:
     {
         struct xen_hvm_set_mem_type a;
-        struct domain *d;
         
         /* Interface types to internal p2m types */
         static const p2m_type_t memtype[] = {
@@ -4500,20 +4498,19 @@ long do_hvm_op(unsigned long op, XEN_GUE
             goto param_fail4;
 
         rc = -EINVAL;
-        if ( ((a.first_pfn + a.nr - 1) < a.first_pfn) ||
+        if ( a.nr < start_iter ||
+             ((a.first_pfn + a.nr - 1) < a.first_pfn) ||
              ((a.first_pfn + a.nr - 1) > domain_get_maximum_gpfn(d)) )
             goto param_fail4;
             
         if ( a.hvmmem_type >= ARRAY_SIZE(memtype) )
             goto param_fail4;
 
-        while ( a.nr )
+        for ( pfn = a.first_pfn + start_iter; a.nr > start_iter; ++pfn )
         {
-            unsigned long pfn = a.first_pfn;
-            p2m_type_t t;
-            p2m_type_t nt;
-            mfn_t mfn;
-            mfn = get_gfn_unshare(d, pfn, &t);
+            p2m_type_t t, nt;
+
+            get_gfn_unshare(d, pfn, &t);
             if ( p2m_is_paging(t) )
             {
                 put_gfn(d, pfn);
@@ -4550,16 +4547,13 @@ long do_hvm_op(unsigned long op, XEN_GUE
             }
             put_gfn(d, pfn);
 
-            a.first_pfn++;
-            a.nr--;
+            ++pfn;
+            ++start_iter;
 
             /* Check for continuation if it's not the last interation */
-            if ( a.nr > 0 && hypercall_preempt_check() )
+            if ( a.nr > start_iter && hypercall_preempt_check() )
             {
-                if ( __copy_to_guest(arg, &a, 1) )
-                    rc = -EFAULT;
-                else
-                    rc = -EAGAIN;
+                rc = -EAGAIN;
                 goto param_fail4;
             }
         }
@@ -4574,7 +4568,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_set_mem_access:
     {
         struct xen_hvm_set_mem_access a;
-        struct domain *d;
 
         if ( copy_from_guest(&a, arg, 1) )
             return -EFAULT;
@@ -4593,19 +4586,17 @@ long do_hvm_op(unsigned long op, XEN_GUE
 
         rc = -EINVAL;
         if ( (a.first_pfn != ~0ull) &&
-             (((a.first_pfn + a.nr - 1) < a.first_pfn) ||
+             (a.nr < start_iter ||
+              ((a.first_pfn + a.nr - 1) < a.first_pfn) ||
               ((a.first_pfn + a.nr - 1) > domain_get_maximum_gpfn(d))) )
             goto param_fail5;
             
-        rc = p2m_set_mem_access(d, a.first_pfn, a.nr, a.hvmmem_access);
+        rc = p2m_set_mem_access(d, a.first_pfn + start_iter, a.nr - start_iter,
+                                a.hvmmem_access);
         if ( rc > 0 )
         {
-            a.first_pfn += a.nr - rc;
-            a.nr = rc;
-            if ( __copy_to_guest(arg, &a, 1) )
-                rc = -EFAULT;
-            else
-                rc = -EAGAIN;
+            start_iter = a.nr - rc;
+            rc = -EAGAIN;
         }
 
     param_fail5:
@@ -4616,7 +4607,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_get_mem_access:
     {
         struct xen_hvm_get_mem_access a;
-        struct domain *d;
         hvmmem_access_t access;
 
         if ( copy_from_guest(&a, arg, 1) )
@@ -4653,7 +4643,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_pagetable_dying:
     {
         struct xen_hvm_pagetable_dying a;
-        struct domain *d;
 
         if ( copy_from_guest(&a, arg, 1) )
             return -EFAULT;
@@ -4706,7 +4695,6 @@ long do_hvm_op(unsigned long op, XEN_GUE
     case HVMOP_inject_trap: 
     {
         xen_hvm_inject_trap_t tr;
-        struct domain *d;
         struct vcpu *v;
 
         if ( copy_from_guest(&tr, arg, 1 ) )
@@ -4754,8 +4742,9 @@ long do_hvm_op(unsigned long op, XEN_GUE
     }
 
     if ( rc == -EAGAIN )
-        rc = hypercall_create_continuation(
-            __HYPERVISOR_hvm_op, "lh", op, arg);
+        rc = hypercall_create_continuation(__HYPERVISOR_hvm_op, "lh",
+                                           op | (start_iter << HVMOP_op_bits),
+                                           arg);
 
     return rc;
 }
--- a/xen/include/public/hvm/hvm_op.h
+++ b/xen/include/public/hvm/hvm_op.h
@@ -90,10 +90,10 @@ typedef enum {
 struct xen_hvm_track_dirty_vram {
     /* Domain to be tracked. */
     domid_t  domid;
+    /* Number of pages to track. */
+    uint32_t nr;
     /* First pfn to track. */
     uint64_aligned_t first_pfn;
-    /* Number of pages to track. */
-    uint64_aligned_t nr;
     /* OUT variable. */
     /* Dirty bitmap buffer. */
     XEN_GUEST_HANDLE_64(uint8) dirty_bitmap;
@@ -106,10 +106,10 @@ DEFINE_XEN_GUEST_HANDLE(xen_hvm_track_di
 struct xen_hvm_modified_memory {
     /* Domain to be updated. */
     domid_t  domid;
+    /* Number of pages. */
+    uint32_t nr;
     /* First pfn. */
     uint64_aligned_t first_pfn;
-    /* Number of pages. */
-    uint64_aligned_t nr;
 };
 typedef struct xen_hvm_modified_memory xen_hvm_modified_memory_t;
 DEFINE_XEN_GUEST_HANDLE(xen_hvm_modified_memory_t);

Attachment: x86-HVM-op-preemption.patch
Description: Text document

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