x86/hvm: split all linear reads and writes at page boundary

Ruling out page straddling at linear level makes it easier to
distinguish chunks that require proper handling as MMIO access
and not complete them as page straddling memory transactions
prematurely. This doesn't change the general behavior.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/emulate.c

@@ -1089,12 +1089,25 @@ static int linear_read(unsigned long addr, unsigned int bytes, void *p_data,
                        uint32_t pfec, struct hvm_emulate_ctxt *hvmemul_ctxt)
 {
     pagefault_info_t pfinfo;
-    int rc = hvm_copy_from_guest_linear(p_data, addr, bytes, pfec, &pfinfo);
+    unsigned int offset = addr & ~PAGE_MASK;
+    int rc;
+
+    if ( offset + bytes > PAGE_SIZE )
+    {
+        unsigned int part1 = PAGE_SIZE - offset;
+
+        /* Split the access at the page boundary. */
+        rc = linear_read(addr, part1, p_data, pfec, hvmemul_ctxt);
+        if ( rc == X86EMUL_OKAY )
+            rc = linear_read(addr + part1, bytes - part1, p_data + part1,
+                             pfec, hvmemul_ctxt);
+        return rc;
+    }
+
+    rc = hvm_copy_from_guest_linear(p_data, addr, bytes, pfec, &pfinfo);
 
     switch ( rc )
     {
-        unsigned int offset, part1;
-
     case HVMTRANS_okay:
         return X86EMUL_OKAY;
 
@@ -1106,19 +1119,9 @@ static int linear_read(unsigned long addr, unsigned int bytes, void *p_data,
         if ( pfec & PFEC_insn_fetch )
             return X86EMUL_UNHANDLEABLE;
 
-        offset = addr & ~PAGE_MASK;
-        if ( offset + bytes <= PAGE_SIZE )
-            return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec,
-                                            hvmemul_ctxt,
-                                            known_gla(addr, bytes, pfec));
-
-        /* Split the access at the page boundary. */
-        part1 = PAGE_SIZE - offset;
-        rc = linear_read(addr, part1, p_data, pfec, hvmemul_ctxt);
-        if ( rc == X86EMUL_OKAY )
-            rc = linear_read(addr + part1, bytes - part1, p_data + part1,
-                             pfec, hvmemul_ctxt);
-        return rc;
+        return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec,
+                                        hvmemul_ctxt,
+                                        known_gla(addr, bytes, pfec));
 
     case HVMTRANS_gfn_paged_out:
     case HVMTRANS_gfn_shared:
@@ -1132,12 +1135,25 @@ static int linear_write(unsigned long addr, unsigned int bytes, void *p_data,
                         uint32_t pfec, struct hvm_emulate_ctxt *hvmemul_ctxt)
 {
     pagefault_info_t pfinfo;
-    int rc = hvm_copy_to_guest_linear(addr, p_data, bytes, pfec, &pfinfo);
+    unsigned int offset = addr & ~PAGE_MASK;
+    int rc;
+
+    if ( offset + bytes > PAGE_SIZE )
+    {
+        unsigned int part1 = PAGE_SIZE - offset;
+
+        /* Split the access at the page boundary. */
+        rc = linear_write(addr, part1, p_data, pfec, hvmemul_ctxt);
+        if ( rc == X86EMUL_OKAY )
+            rc = linear_write(addr + part1, bytes - part1, p_data + part1,
+                              pfec, hvmemul_ctxt);
+        return rc;
+    }
+
+    rc = hvm_copy_to_guest_linear(addr, p_data, bytes, pfec, &pfinfo);
 
     switch ( rc )
     {
-        unsigned int offset, part1;
-
     case HVMTRANS_okay:
         return X86EMUL_OKAY;
 
@@ -1146,19 +1162,9 @@ static int linear_write(unsigned long addr, unsigned int bytes, void *p_data,
         return X86EMUL_EXCEPTION;
 
     case HVMTRANS_bad_gfn_to_mfn:
-        offset = addr & ~PAGE_MASK;
-        if ( offset + bytes <= PAGE_SIZE )
-            return hvmemul_linear_mmio_write(addr, bytes, p_data, pfec,
-                                             hvmemul_ctxt,
-                                             known_gla(addr, bytes, pfec));
-
-        /* Split the access at the page boundary. */
-        part1 = PAGE_SIZE - offset;
-        rc = linear_write(addr, part1, p_data, pfec, hvmemul_ctxt);
-        if ( rc == X86EMUL_OKAY )
-            rc = linear_write(addr + part1, bytes - part1, p_data + part1,
-                              pfec, hvmemul_ctxt);
-        return rc;
+        return hvmemul_linear_mmio_write(addr, bytes, p_data, pfec,
+                                         hvmemul_ctxt,
+                                         known_gla(addr, bytes, pfec));
 
     case HVMTRANS_gfn_paged_out:
     case HVMTRANS_gfn_shared: