Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 14 Sep 2015 10:55:53 -0700 | Subject | Re: [PATCH v4 RESEND] x86/asm/entry/32, selftests: Add 'test_syscall_vdso' test |
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On Sep 14, 2015 1:15 AM, "Ingo Molnar" <mingo@kernel.org> wrote: > > > * Denys Vlasenko <dvlasenk@redhat.com> wrote: > > > >> + /* INT80 syscall entrypoint can be used by > > >> + * 64-bit programs too, unlike SYSCALL/SYSENTER. > > >> + * Therefore it must preserve R12+ > > >> + * (they are callee-saved registers in 64-bit C ABI). > > >> + * > > >> + * This was probably historically not intended, > > >> + * but R8..11 are clobbered (cleared to 0). > > >> + * IOW: they are the only registers which aren't > > >> + * preserved across INT80 syscall. > > >> + */ > > >> + if (*r64 == 0 && num <= 11) > > >> + continue; > > > > > > Ugh. I'll change my big entry patchset to preserve these and maybe to > > > preserve all of the 64-bit regs. > > > > If you do that, this won't change the ABI: we don't _promise_ > > to save them. If we accidentally do, that means nothing. > > Argh, that's dangerous nonsense! You _still_ don't seem to understand what the > Linux ABI means and how to change code that implements it...
I think Denys might be taking about R8-R11 here. If we change them from clobbered to saved, that's probably fine. Certainly I have to save R12-R15 -- my v1 is just buggy there. I was too deep in __kernel_vsyscall when I wrote that code, and I wasn't thinking about the raw int $0x80 entry variant.
I'd be rather surprised if anything broke if we started preserving R8-R11 instead of zeroing them.
--Andy
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