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

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jun 9, 2015 at 12:18 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 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.

I suppose that's true enough even if it's not quite true.  A 64-bit
task could far jump/call/return to compat mode and then do SYSCALL or
SYSENTER, but it will likely crash and burn because there's no 32-bit
vdso.

--Andy