Re: [PATCH 4/5] x86/asm/entry/32: Replace RESTORE_RSI_RDI[_RDX] with open-coded 32-bit reads

Denys Vlasenko
2015-06-09T19:18:14Z

On 06/09/2015 09:11 PM, Andy Lutomirski wrote:
> On Tue, Jun 9, 2015 at 12:03 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>> On 06/09/2015 09:01 PM, Andy Lutomirski wrote:
>>> On Tue, Jun 9, 2015 at 11:54 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>>> This doesn't change much, but this uses shorter 32-bit insns:
>>>>
>>>> -48 8b 74 24 68 mov 0x68(%rsp),%rsi
>>>> -48 8b 7c 24 70 mov 0x70(%rsp),%rdi
>>>> -48 8b 54 24 60 mov 0x60(%rsp),%rdx
>>>> +8b 74 24 68 mov 0x68(%rsp),%esi
>>>> +8b 7c 24 70 mov 0x70(%rsp),%edi
>>>> +8b 54 24 60 mov 0x60(%rsp),%edx
>>>>
>>>> Since these are the only uses of RESTORE_RSI_RDI[_RDX], drop these macros.
>>>>
>>>
>>> It probably doesn't matter for these fast paths, but, for the full
>>> slow path return, we really do need to restore the full pt_regs.
>>> After all, the syscall we're returning from might be sigreturn.
>>
>> This is compat 32-bit syscall handling code.
>> IIUC we do not restore high half of any registers for 32-bit tasks.
>>
>> Am I missing something?
>
> Yes -- 64-bit tasks can call 32-bit compat syscalls.

Not via SYSCALL and SYSENTER code paths. This patch touches only those
code paths.