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

[Denys Vlasenko]

On 06/09/2015 09:11 PM, Andy Lutomirski wrote:
> On Tue, Jun 9, 2015 at 12:03 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>> On 06/09/2015 09:01 PM, Andy Lutomirski wrote:
>>> On Tue, Jun 9, 2015 at 11:54 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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.
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