Re: [kvm-unit-tests PATCH] Fixes for the umip test

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Thomas Huth <thuth@redhat.com> writes:

> 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.
>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  x86/umip.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/x86/umip.c b/x86/umip.c
> index 7eee294..834668c 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 +160,7 @@ 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 -  2]),

My eyes would definitely appreciate the missing parentheses and the
absense of the redundant space:

  sizeof(user_stack) - 2

>  		    [fn]"r"(fn),
>  		    [arg]"D"(arg),
>  		    [kernel_ds]"i"(KERNEL_DS),

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly