Re: [Xen-devel] [PATCH v7 08/14] x86: add multiboot2 protocol support for EFI platforms

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/09/16 15:33, Jan Beulich wrote:
>>>> On 26.09.16 at 16:19, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 23/09/16 22:47, Daniel Kiper wrote:
>>> +        /*
>>> +         * Initialize BSS (no nasty surprises!).
>>> +         * It must be done earlier than in BIOS case
>>> +         * because efi_multiboot2() touches it.
>>> +         */
>>> +        lea     .startof.(.bss)(%rip),%edi
>>> +        mov     $.sizeof.(.bss),%ecx
>> Sorry, but you cannot use this syntax, for the same reasons why I won't
>> accept Jan's patch making similar changes elsewhere.
>>
>> Amongst other issues, you will break the Clang build with it.
> Did you verify meanwhile that this doesn't work with llvm?

Yes.

andrewcoop@andrewcoop:/local/xen.git/xen$ cat foo.c
#include <stdio.h>

static unsigned int x;

int main(void)
{
    asm volatile("lea .startof.(.bss)(%%rip), %%rdi;"
                 "mov $.sizeof.(.bss), %%rcx;"
                 "mov $-1, %%rax;"
                 "rep stosb;"
                 ::: "memory", "rax", "rcx", "rdi");
    printf("x: %#x\n", x);
}
andrewcoop@andrewcoop:/local/xen.git/xen$ gcc foo.c -o foo && ./foo
x: 0xffffffff
andrewcoop@andrewcoop:/local/xen.git/xen$ clang-3.8 foo.c -o foo && ./foo
foo.c:7:18: error: unexpected token in memory operand
    asm volatile("lea .startof.(.bss)(%%rip), %%rdi;"
                 ^
<inline asm>:1:16: note: instantiated into assembly here
        lea .startof.(.bss)(%rip), %rdi;mov $.sizeof.(.bss), %rcx;mov
$-1, %rax;rep stosb;
                      ^
foo.c:7:18: error: unexpected token in argument list
    asm volatile("lea .startof.(.bss)(%%rip), %%rdi;"
                 ^
<inline asm>:1:47: note: instantiated into assembly here
        lea .startof.(.bss)(%rip), %rdi;mov $.sizeof.(.bss), %rcx;mov
$-1, %rax;rep stosb;
                                                     ^
2 errors generated.

~Andrew

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