Subject: Re: [RFC PATCH 1/2] powerpc: Add a proper syscall for switching endianness

From: Anshuman Khandual <[email-masked]>
Date: Wed, 11 Mar 2015 12:02:15 +0530

On 03/11/2015 11:13 AM, Benjamin Herrenschmidt wrote:
> On Wed, 2015-03-11 at 11:08 +0530, Anshuman Khandual wrote:
>> On 03/10/2015 04:25 PM, Michael Ellerman wrote:
>>> On Tue, 2015-03-10 at 20:34 +1100, Benjamin Herrenschmidt wrote:
>>>> On Tue, 2015-03-10 at 18:36 +1100, Michael Ellerman wrote:
>>>>> We currently have a "special" syscall for switching endianness. This is
>>>>> syscall number 0x1ebe, which is handled explicitly in the 64-bit syscall
>>>>> exception entry.
>>>>>
>>>>> That has a few problems, firstly the syscall number is outside of the
>>>>> usual range, which confuses various tools. For example strace doesn't
>>>>> recognise the syscalls at all.
>>>>>
>>>>> Secondly it's handled explicitly as a special case in the syscall
>>>>> exception entry, which is complicated enough without it.
>>>>>
>>>>> As a first step toward removing the special syscall, we need to add a
>>>>> regular syscall that implements the same functionality.
>>>>>
>>>>> The logic is simple, it simply toggles the MSR_LE bit in the userspace
>>>>> MSR. This is the same as the special syscall, with the caveat that the
>>>>> special syscall clobbers fewer registers.
>>>>
>>>> You can set _TIF_RESTOREALL to force a restore of all the registers on
>>>> the way back which should do the job.
>>>
>>> Right, I'd forgotten we talked about that.
>>>
>>> I'll try that tomorrow.
>>
>> The test fails when we add set_thread_flag(TIF_RESTOREALL) after the MSR flip.
>> Though the test passes with the original patch.
> 
> We also need to wrap the syscall like we do for fork() etc... to
> save_nvgpr's.

Yeah right, replacing SYSCALL(switch_endian) with PPC_SYS(switch_endian)
in systbl.h does the trick and the test passes again.