x86/mce: Reduce number of machine checks taken during recovery

When any of the copy functions in arch/x86/lib/copy_user_64.S take a
fault, the fixup code copies the remaining byte count from %ecx to %edx
and unconditionally jumps to .Lcopy_user_handle_tail to continue the
copy in case any more bytes can be copied.

If the fault was #PF this may copy more bytes (because the page fault
handler might have fixed the fault). But when the fault is a machine
check the original copy code will have copied all the way to the poisoned
cache line. So .Lcopy_user_handle_tail will just take another machine
check for no good reason.

Every code path to .Lcopy_user_handle_tail comes from an exception fixup
path, so add a check there to check the trap type (in %eax) and simply
return the count of remaining bytes if the trap was a machine check.

Doing this reduces the number of machine checks taken during synthetic
tests from four to three.

As well as reducing the number of machine checks, this also allows
Skylake generation Xeons to recover some cases that currently fail. The
is because REP; MOVSB is only recoverable when source and destination
are well aligned and the byte count is large. That useless call to
.Lcopy_user_handle_tail may violate one or more of these conditions and
generate a fatal machine check.

  [ Tony: Add more details to commit message. ]
  [ bp: Fixup comment.
    Also, another tip patchset which is adding straight-line speculation
    mitigation changes the "ret" instruction to an all-caps macro "RET".
    But, since gas is case-insensitive, use "RET" in the newly added asm block
    already in order to simplify tip branch merging on its way upstream.
  ]
Signed-off-by: Youquan Song <youquan.song@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/YcTW5dh8yTGucDd+@agluck-desk2.amr.corp.intel.com

arch/x86/lib/copy_user_64.S

@@ -225,6 +225,7 @@ EXPORT_SYMBOL(copy_user_enhanced_fast_string)
  * Don't try to copy the tail if machine check happened
  *
  * Input:
+ * eax trap number written by ex_handler_copy()
  * rdi destination
  * rsi source
  * rdx count
@@ -233,12 +234,20 @@ EXPORT_SYMBOL(copy_user_enhanced_fast_string)
  * eax uncopied bytes or 0 if successful.
  */
 SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
+	cmp $X86_TRAP_MC,%eax
+	je 3f
+
 	movl %edx,%ecx
 1:	rep movsb
 2:	mov %ecx,%eax
 	ASM_CLAC
 	ret
 
+3:
+	movl %edx,%eax
+	ASM_CLAC
+	RET
+
 	_ASM_EXTABLE_CPY(1b, 2b)
 SYM_CODE_END(.Lcopy_user_handle_tail)