[PATCH] x86/PV: simplify (and thus correct) guest accessor functions

[Jan Beulich <jbeulich@xxxxxxxx>]

Taking a fault on a non-byte-granular insn means that the "number of
bytes not handled" return value would need extra care in calculating, if
we want callers to be able to derive e.g. exception context (to be
injected to the guest) - CR2 for #PF in particular - from the value. To
simplify things rather than complicating them, reduce inline assembly to
just byte-granular string insns. On recent CPUs that's also supposed to
be more efficient anyway.

For singular element accessors, however, alignment checks are added,
hence slightly complicating the code. Misaligned (user) buffer accesses
will now be forwarded to copy_{from,to}_guest_ll().

Naturally copy_{from,to}_unsafe_ll() accessors end up being adjusted the
same way, as they're produced by mere re-processing of the same code.
Otoh copy_{from,to}_unsafe() aren't similarly adjusted, but have their
comments made match reality; down the road we may want to change their
return types, e.g. to bool.

Fixes: 76974398a63c ("Added user-memory accessing functionality for x86_64")
Fixes: 7b8c36701d26 ("Introduce clear_user and clear_guest")
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/include/asm/uaccess.h
+++ b/xen/arch/x86/include/asm/uaccess.h
@@ -251,7 +251,8 @@ do {
 static always_inline unsigned long
 __copy_to_guest_pv(void __user *to, const void *from, unsigned long n)
 {
-    if (__builtin_constant_p(n)) {
+    if ( __builtin_constant_p(n) && !((unsigned long)to & (n - 1)) )
+    {
         unsigned long ret;
 
         switch (n) {
@@ -291,7 +292,8 @@ __copy_to_guest_pv(void __user *to, cons
 static always_inline unsigned long
 __copy_from_guest_pv(void *to, const void __user *from, unsigned long n)
 {
-    if (__builtin_constant_p(n)) {
+    if ( __builtin_constant_p(n) && !((unsigned long)from & (n - 1)) )
+    {
         unsigned long ret;
 
         switch (n) {
@@ -321,8 +323,7 @@ __copy_from_guest_pv(void *to, const voi
  *
  * Copy data from hypervisor space to a potentially unmapped area.
  *
- * Returns number of bytes that could not be copied.
- * On success, this will be zero.
+ * Returns zero on success and non-zero if some bytes could not be copied.
  */
 static always_inline unsigned int
 copy_to_unsafe(void __user *to, const void *from, unsigned int n)
@@ -358,8 +359,7 @@ copy_to_unsafe(void __user *to, const vo
  *
  * Copy data from a potentially unmapped area space to hypervisor space.
  *
- * Returns number of bytes that could not be copied.
- * On success, this will be zero.
+ * Returns zero on success and non-zero if some bytes could not be copied.
  *
  * If some data could not be copied, this function will pad the copied
  * data to the requested size using zero bytes.
--- a/xen/arch/x86/usercopy.c
+++ b/xen/arch/x86/usercopy.c
@@ -16,42 +16,19 @@
 
 unsigned int copy_to_guest_ll(void __user *to, const void *from, unsigned int 
n)
 {
-    unsigned dummy;
+    GUARD(unsigned dummy);
 
     stac();
     asm volatile (
         GUARD(
         "    guest_access_mask_ptr %[to], %q[scratch1], %q[scratch2]\n"
         )
-        "    cmp  $"STR(2*BYTES_PER_LONG-1)", %[cnt]\n"
-        "    jbe  1f\n"
-        "    mov  %k[to], %[cnt]\n"
-        "    neg  %[cnt]\n"
-        "    and  $"STR(BYTES_PER_LONG-1)", %[cnt]\n"
-        "    sub  %[cnt], %[aux]\n"
-        "4:  rep movsb\n" /* make 'to' address aligned */
-        "    mov  %[aux], %[cnt]\n"
-        "    shr  $"STR(LONG_BYTEORDER)", %[cnt]\n"
-        "    and  $"STR(BYTES_PER_LONG-1)", %[aux]\n"
-        "    .align 2,0x90\n"
-        "0:  rep movs"__OS"\n" /* as many words as possible... */
-        "    mov  %[aux],%[cnt]\n"
-        "1:  rep movsb\n" /* ...remainder copied as bytes */
+        "1:  rep movsb\n"
         "2:\n"
-        ".section .fixup,\"ax\"\n"
-        "5:  add %[aux], %[cnt]\n"
-        "    jmp 2b\n"
-        "3:  lea (%q[aux], %q[cnt], "STR(BYTES_PER_LONG)"), %[cnt]\n"
-        "    jmp 2b\n"
-        ".previous\n"
-        _ASM_EXTABLE(4b, 5b)
-        _ASM_EXTABLE(0b, 3b)
         _ASM_EXTABLE(1b, 2b)
-        : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from),
-          [aux] "=&r" (dummy)
+        : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from)
           GUARD(, [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy))
-        : "[aux]" (n)
-        : "memory" );
+        :: "memory" );
     clac();
 
     return n;
@@ -66,25 +43,9 @@ unsigned int copy_from_guest_ll(void *to
         GUARD(
         "    guest_access_mask_ptr %[from], %q[scratch1], %q[scratch2]\n"
         )
-        "    cmp  $"STR(2*BYTES_PER_LONG-1)", %[cnt]\n"
-        "    jbe  1f\n"
-        "    mov  %k[to], %[cnt]\n"
-        "    neg  %[cnt]\n"
-        "    and  $"STR(BYTES_PER_LONG-1)", %[cnt]\n"
-        "    sub  %[cnt], %[aux]\n"
-        "4:  rep movsb\n" /* make 'to' address aligned */
-        "    mov  %[aux],%[cnt]\n"
-        "    shr  $"STR(LONG_BYTEORDER)", %[cnt]\n"
-        "    and  $"STR(BYTES_PER_LONG-1)", %[aux]\n"
-        "    .align 2,0x90\n"
-        "0:  rep movs"__OS"\n" /* as many words as possible... */
-        "    mov  %[aux], %[cnt]\n"
-        "1:  rep movsb\n" /* ...remainder copied as bytes */
+        "1:  rep movsb\n"
         "2:\n"
         ".section .fixup,\"ax\"\n"
-        "5:  add  %[aux], %[cnt]\n"
-        "    jmp 6f\n"
-        "3:  lea  (%q[aux], %q[cnt], "STR(BYTES_PER_LONG)"), %[cnt]\n"
         "6:  mov  %[cnt], %k[from]\n"
         "    xchg %%eax, %[aux]\n"
         "    xor  %%eax, %%eax\n"
@@ -93,14 +54,11 @@ unsigned int copy_from_guest_ll(void *to
         "    mov  %k[from], %[cnt]\n"
         "    jmp 2b\n"
         ".previous\n"
-        _ASM_EXTABLE(4b, 5b)
-        _ASM_EXTABLE(0b, 3b)
         _ASM_EXTABLE(1b, 6b)
         : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from),
           [aux] "=&r" (dummy)
           GUARD(, [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy))
-        : "[aux]" (n)
-        : "memory" );
+        :: "memory" );
     clac();
 
     return n;
@@ -145,20 +103,12 @@ unsigned int clear_guest_pv(void __user
         stac();
         asm volatile (
             "    guest_access_mask_ptr %[to], %[scratch1], %[scratch2]\n"
-            "0:  rep stos"__OS"\n"
-            "    mov  %[bytes], %[cnt]\n"
             "1:  rep stosb\n"
             "2:\n"
-            ".section .fixup,\"ax\"\n"
-            "3:  lea  (%q[bytes], %q[longs], "STR(BYTES_PER_LONG)"), %[cnt]\n"
-            "    jmp  2b\n"
-            ".previous\n"
-            _ASM_EXTABLE(0b,3b)
             _ASM_EXTABLE(1b,2b)
-            : [cnt] "=&c" (n), [to] "+D" (to), [scratch1] "=&r" (dummy),
+            : [cnt] "+c" (n), [to] "+D" (to), [scratch1] "=&r" (dummy),
               [scratch2] "=&r" (dummy)
-            : [bytes] "r" (n & (BYTES_PER_LONG - 1)),
-              [longs] "0" (n / BYTES_PER_LONG), "a" (0) );
+            : "a" (0) );
         clac();
     }