Re: [Qemu-devel] [PATCH] linux-user: Use correct alignment for long long on i386 guests

[Laurent Vivier]

Le 28/07/2016 à 13:57, Peter Maydell a écrit :
> For i386, the ABI specifies that 'long long' (8 byte values)
> need only be 4 aligned, but we were requiring them to be
> 8-aligned. This meant we were laying out the target_epoll_event
> structure wrongly. Add a suitable ifdef to abitypes.h to
> specify the i386-specific alignment requirement.

gdb qemu-i386
(gdb) p &(((struct target_epoll_event *)0)->data)
$1 = (target_epoll_data_t *) 0x8

whereas:

gdb qemu-x86_64
(gdb) p &(((struct target_epoll_event *)0)->data)
$1 = (target_epoll_data_t *) 0x4

I've checked on real systems x86_64/i386:
-----
#include <sys/epoll.h>

int main(void)
{
    volatile struct epoll_event e;

    e.events = 0;
}
----
(gdb) p &(((struct epoll_event *)0)->data)
$1 = (epoll_data_t *) 0x4

but on ppc64, I have

(gdb) p &(((struct epoll_event *)0)->data)
$1 = (epoll_data_t *) 0x8

In fact, the structure should be packed in both cases, something like:

--- a/linux-user/syscall_defs.h
+++ b/linux-user/syscall_defs.h
@@ -2562,7 +2562,7 @@ struct target_mq_attr {
 #define FUTEX_CMD_MASK          ~(FUTEX_PRIVATE_FLAG |
FUTEX_CLOCK_REALTIME)

 #ifdef CONFIG_EPOLL
-#if defined(TARGET_X86_64)
+#if defined(TARGET_X86_64) || defined(TARGET_I386)
 #define TARGET_EPOLL_PACKED QEMU_PACKED
 #else
 #define TARGET_EPOLL_PACKED

on my Fedora systems x86_64/i386:

/usr/include/bits/epoll.h

#define __EPOLL_PACKED __attribute__ ((__packed__))

/usr/include/sys/epoll.h

struct epoll_event
{
  uint32_t events;      /* Epoll events */
  epoll_data_t data;    /* User data variable */
} __EPOLL_PACKED;

but I don't understand why in linux source tree we have

#ifdef __x86_64__
#define EPOLL_PACKED __attribute__((packed))
#else
#define EPOLL_PACKED
#endif

struct epoll_event {
        __u32 events;
        __u64 data;
} EPOLL_PACKED;

Laurent