[PATCH v5 1/5] x86/HVM: split restore state checking from state loading

[Jan Beulich <jbeulich@xxxxxxxx>]

..., at least as reasonably feasible without making a check hook
mandatory (in particular strict vs relaxed/zero-extend length checking
can't be done early this way).

Note that only one of the two uses of "real" hvm_load() is accompanied
with a "checking" one. The other directly consumes hvm_save() output,
which ought to be well-formed. This means that while input data related
checks don't need repeating in the "load" function when already done by
the "check" one (albeit assertions to this effect may be desirable),
domain state related checks (e.g. has_xyz(d)) will be required in both
places.

With the split arch_hvm_{check,load}(), also invoke the latter only
after downing all the vCPU-s.

Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Do we really need all the copying involved in use of _hvm_read_entry()
(backing hvm_load_entry()? Zero-extending loads are likely easier to
handle that way, but for strict loads all we gain is a reduced risk of
unaligned accesses (compared to simply pointing into h->data[]).
---
v5: Add comment. Re-order vCPU-down vs arch_hvm_load().
v4: Fold hvm_check() into hvm_load().
v2: New.

--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -379,8 +379,12 @@ long arch_do_domctl(
         if ( copy_from_guest(c.data, domctl->u.hvmcontext.buffer, c.size) != 0 
)
             goto sethvmcontext_out;
 
+        ret = hvm_load(d, false, &c);
+        if ( ret )
+            goto sethvmcontext_out;
+
         domain_pause(d);
-        ret = hvm_load(d, &c);
+        ret = hvm_load(d, true, &c);
         domain_unpause(d);
 
     sethvmcontext_out:
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -5391,7 +5391,7 @@ int hvm_copy_context_and_params(struct d
     }
 
     c.cur = 0;
-    rc = hvm_load(dst, &c);
+    rc = hvm_load(dst, true, &c);
 
  out:
     vfree(c.data);
--- a/xen/arch/x86/hvm/save.c
+++ b/xen/arch/x86/hvm/save.c
@@ -30,7 +30,8 @@ static void arch_hvm_save(struct domain
     d->arch.hvm.sync_tsc = rdtsc();
 }
 
-static int arch_hvm_load(struct domain *d, const struct hvm_save_header *hdr)
+static int arch_hvm_check(const struct domain *d,
+                          const struct hvm_save_header *hdr)
 {
     uint32_t eax, ebx, ecx, edx;
 
@@ -55,6 +56,11 @@ static int arch_hvm_load(struct domain *
                "(%#"PRIx32") and restored on another (%#"PRIx32").\n",
                d->domain_id, hdr->cpuid, eax);
 
+    return 0;
+}
+
+static void arch_hvm_load(struct domain *d, const struct hvm_save_header *hdr)
+{
     /* Restore guest's preferred TSC frequency. */
     if ( hdr->gtsc_khz )
         d->arch.tsc_khz = hdr->gtsc_khz;
@@ -66,13 +72,12 @@ static int arch_hvm_load(struct domain *
 
     /* VGA state is not saved/restored, so we nobble the cache. */
     d->arch.hvm.stdvga.cache = STDVGA_CACHE_DISABLED;
-
-    return 0;
 }
 
 /* List of handlers for various HVM save and restore types */
 static struct {
     hvm_save_handler save;
+    hvm_check_handler check;
     hvm_load_handler load;
     const char *name;
     size_t size;
@@ -88,6 +93,7 @@ void __init hvm_register_savevm(uint16_t
 {
     ASSERT(typecode <= HVM_SAVE_CODE_MAX);
     ASSERT(hvm_sr_handlers[typecode].save == NULL);
+    ASSERT(hvm_sr_handlers[typecode].check == NULL);
     ASSERT(hvm_sr_handlers[typecode].load == NULL);
     hvm_sr_handlers[typecode].save = save_state;
     hvm_sr_handlers[typecode].load = load_state;
@@ -275,12 +281,15 @@ int hvm_save(struct domain *d, hvm_domai
     return 0;
 }
 
-int hvm_load(struct domain *d, hvm_domain_context_t *h)
+/*
+ * @real = false requests checking of the incoming state, while @real = true
+ * requests actual loading, which will then assume that checking was already
+ * done or is unnecessary.
+ */
+int hvm_load(struct domain *d, bool real, hvm_domain_context_t *h)
 {
     const struct hvm_save_header *hdr;
     struct hvm_save_descriptor *desc;
-    hvm_load_handler handler;
-    struct vcpu *v;
     int rc;
 
     if ( d->is_dying )
@@ -291,50 +300,92 @@ int hvm_load(struct domain *d, hvm_domai
     if ( !hdr )
         return -ENODATA;
 
-    rc = arch_hvm_load(d, hdr);
-    if ( rc )
-        return rc;
+    rc = arch_hvm_check(d, hdr);
+    if ( real )
+    {
+        struct vcpu *v;
+
+        ASSERT(!rc);
 
-    /* Down all the vcpus: we only re-enable the ones that had state saved. */
-    for_each_vcpu(d, v)
-        if ( !test_and_set_bit(_VPF_down, &v->pause_flags) )
-            vcpu_sleep_nosync(v);
+        /*
+         * Down all the vcpus: we only re-enable the ones that had state
+         * saved.
+         */
+        for_each_vcpu(d, v)
+            if ( !test_and_set_bit(_VPF_down, &v->pause_flags) )
+                vcpu_sleep_nosync(v);
+
+        arch_hvm_load(d, hdr);
+    }
+    else if ( rc )
+        return rc;
 
     for ( ; ; )
     {
+        const char *name;
+        hvm_load_handler load;
+
         if ( h->size - h->cur < sizeof(struct hvm_save_descriptor) )
         {
             /* Run out of data */
             printk(XENLOG_G_ERR
                    "HVM%d restore: save did not end with a null entry\n",
                    d->domain_id);
+            ASSERT(!real);
             return -ENODATA;
         }
 
         /* Read the typecode of the next entry  and check for the end-marker */
         desc = (struct hvm_save_descriptor *)(&h->data[h->cur]);
-        if ( desc->typecode == 0 )
+        if ( desc->typecode == HVM_SAVE_CODE(END) )
+        {
+            /* Reset cursor for hvm_load(, true, ). */
+            if ( !real )
+                h->cur = 0;
             return 0;
+        }
 
         /* Find the handler for this entry */
-        if ( (desc->typecode > HVM_SAVE_CODE_MAX) ||
-             ((handler = hvm_sr_handlers[desc->typecode].load) == NULL) )
+        if ( desc->typecode >= ARRAY_SIZE(hvm_sr_handlers) ||
+             !(name = hvm_sr_handlers[desc->typecode].name) ||
+             !(load = hvm_sr_handlers[desc->typecode].load) )
         {
             printk(XENLOG_G_ERR "HVM%d restore: unknown entry typecode %u\n",
                    d->domain_id, desc->typecode);
+            ASSERT(!real);
             return -EINVAL;
         }
 
-        /* Load the entry */
-        printk(XENLOG_G_INFO "HVM%d restore: %s %"PRIu16"\n", d->domain_id,
-               hvm_sr_handlers[desc->typecode].name, desc->instance);
-        rc = handler(d, h);
+        if ( real )
+        {
+            /* Load the entry */
+            printk(XENLOG_G_INFO "HVM restore %pd: %s %"PRIu16"\n", d,
+                   name, desc->instance);
+            rc = load(d, h);
+        }
+        else
+        {
+            /* Check the entry. */
+            hvm_check_handler check = hvm_sr_handlers[desc->typecode].check;
+
+            if ( !check )
+            {
+                if ( desc->length > h->size - h->cur - sizeof(*desc) )
+                    return -ENODATA;
+                h->cur += sizeof(*desc) + desc->length;
+                rc = 0;
+            }
+            else
+                rc = check(d, h);
+        }
+
         if ( rc )
         {
-            printk(XENLOG_G_ERR "HVM%d restore: failed to load entry %u/%u rc 
%d\n",
-                   d->domain_id, desc->typecode, desc->instance, rc);
+            printk(XENLOG_G_ERR "HVM restore %pd: failed to %s %s:%u rc %d\n",
+                   d, real ? "load" : "check", name, desc->instance, rc);
             return rc;
         }
+
         process_pending_softirqs();
     }
 
--- a/xen/arch/x86/include/asm/hvm/save.h
+++ b/xen/arch/x86/include/asm/hvm/save.h
@@ -103,6 +103,8 @@ static inline unsigned int hvm_load_inst
  * restoring.  Both return non-zero on error. */
 typedef int (*hvm_save_handler) (struct vcpu *v,
                                  hvm_domain_context_t *h);
+typedef int (*hvm_check_handler)(const struct domain *d,
+                                 hvm_domain_context_t *h);
 typedef int (*hvm_load_handler) (struct domain *d,
                                  hvm_domain_context_t *h);
 
@@ -140,6 +142,6 @@ size_t hvm_save_size(struct domain *d);
 int hvm_save(struct domain *d, hvm_domain_context_t *h);
 int hvm_save_one(struct domain *d, unsigned int typecode, unsigned int 
instance,
                  XEN_GUEST_HANDLE_64(uint8) handle, uint64_t *bufsz);
-int hvm_load(struct domain *d, hvm_domain_context_t *h);
+int hvm_load(struct domain *d, bool real, hvm_domain_context_t *h);
 
 #endif /* __XEN_HVM_SAVE_H__ */