Messages in this thread | | | Date | Sun, 19 Feb 2012 16:56:08 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 02/30] x86-64: Use explicit sizes in sigcontext.h, prepare for x32 |
| |
On 02/19/2012 04:51 PM, Linus Torvalds wrote: > On Sun, Feb 19, 2012 at 4:07 PM, H. Peter Anvin <hpa@zytor.com> wrote: >> >> Use explicit sizes (__u64) instead of implicit sizes (unsigned long) >> in the definition for sigcontext.h; this will allow this structure to >> be shared between the x86-64 native ABI and the x32 ABI. > > Btw, since we had this issue just with autofs: what are the x32 ABI > alignment issues for __u64? Are they like x86-64 ("natural alignment") > or x86-32 ("4-byte alignment")? > > I assume they are natural alignment, and as pointed out by Davem, we > do have the versions of u64 that make this explicit: "compat_u64" is > the 4-byte-aligned one, while "__aligned_u64" is the natively aligned > one. > > Just plain "__u64" doesn't tell which it is, which is sad and wrong, > but we're likely stuck with it forever. Unless some shining knight > comes and says "__u64 is native alignment, and if you want anything > else, you need to use __compat_u64", and actually fixes the cases > where x86-32 depends on the 4-byte aligned one. > > Which would be nice, but sounds unlikely. Shining knights tend to be > rare. But this *could* possibly be automated, so it's not entirely out > of the question. >
We are using __u64 as x86-32 compatible since we are sharing most of the really complex path (like ioctl) with i386 much more so than x86-64. So it is defined in userspace as:
typedef unsigned long long __u64 __attribute__((aligned(4)));
__aligned_u64 obviously is naturally aligned, which matches uint64_t is userspace.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
| |