Fixes for the umip test

When compiling umip.c with -O2 instead of -O1, there are currently
two problems. First, the compiler complains:

 x86/umip.c: In function ‘do_ring3’:
 x86/umip.c:162:37: error: array subscript 4096 is above array bounds of
    ‘unsigned char[4096]’ [-Werror=array-bounds]
       [user_stack_top]"m"(user_stack[sizeof user_stack]),
                           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

This can be fixed by initializing the stack to point to one of the last
bytes of the array instead.

The second problem is that some tests are failing - and this is due
to the fact that the GP_ASM macro uses inline asm without the "volatile"
keyword - so that the compiler reorders this code in certain cases
where it should not. Fix it by adding "volatile" here.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200512094438.17998-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/x86/umip.c b/x86/umip.c
index afb373d..c5700b3 100644
--- a/x86/umip.c
+++ b/x86/umip.c
@@ -22,7 +22,8 @@ static void gp_handler(struct ex_regs *regs)
 
 
 #define GP_ASM(stmt, in, clobber)                  \
-     asm ("mov" W " $1f, %[expected_rip]\n\t"      \
+    asm volatile (                                 \
+          "mov" W " $1f, %[expected_rip]\n\t"      \
           "movl $2f-1f, %[skip_count]\n\t"         \
           "1: " stmt "\n\t"                        \
           "2: "                                    \
@@ -159,7 +160,8 @@ static int do_ring3(void (*fn)(const char *), const char *arg)
 		  : [ret] "=&a" (ret)
 		  : [user_ds] "i" (USER_DS),
 		    [user_cs] "i" (USER_CS),
-		    [user_stack_top]"m"(user_stack[sizeof user_stack]),
+		    [user_stack_top]"m"(user_stack[sizeof(user_stack) -
+						   sizeof(long)]),
 		    [fn]"r"(fn),
 		    [arg]"D"(arg),
 		    [kernel_ds]"i"(KERNEL_DS),