[PATCH] lib: sbi_illegal_insn: Emulate 'MZ'/c.li s4,-13 instruction

[Jessica Clarke]

On 9 Nov 2023, at 07:10, Xiang W <wxjstz at 126.com> wrote:
> 在 2023-11-09星期四的 07:00 +0000，Jessica Clarke写道：
>> On 9 Nov 2023, at 06:55, Xiang W <wxjstz at 126.com> wrote:
>>> 在 2023-11-09星期四的 06:24 +0000，Jessica Clarke写道：
>>>> On 9 Nov 2023, at 04:04, Xiang W <wxjstz at 126.com> wrote:
>>>>> 
>>>>> 在 2023-11-09星期四的 01:22 +0100，Björn Töpel写道：
>>>>>> On Thu, 9 Nov 2023 at 01:11, Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>>>>>>> 
>>>>>>> On 9 Nov 2023, at 00:02, Björn Töpel <bjorn at kernel.org> wrote:
>>>>>>>> 
>>>>>>>> Hi Andreas!
>>>>>>>> 
>>>>>>>> On Wed, 8 Nov 2023 at 22:55, Andreas Schwab <schwab at linux-m68k.org> wrote:
>>>>>>>>> 
>>>>>>>>> On Nov 08 2023, Björn Töpel wrote:
>>>>>>>>> 
>>>>>>>>>> +static int compressed_insn(ulong insn, struct sbi_trap_regs *regs)
>>>>>>>>>> +{
>>>>>>>>>> +     /* Only handle 'MZ'/c.li s4,-13/0x5a3d */
>>>>>>>>>> +     if (!misa_extension('C') && (insn & 0xffff) == 0x5a4d) {
>>>>>>>>>> +             regs->s4 = -13;
>>>>>>>>>> +             regs->mepc += 4;
>>>>>>>>> 
>>>>>>>>> By skipping 4 bytes execution will resume in the middle of the next insn
>>>>>>>>> (the jump around the header).
>>>>>>>> 
>>>>>>>> This is in a non-C environment -- "!misa_extension('C')", so we're not
>>>>>>>> jumping into the middle of the next insn.
>>>>>>> 
>>>>>>> But that’s not c.li, is it. That’s bjorn.c.li, a 32-bit instruction
>>>>>>                                                  ^^^^ My very own! :-D
>>>>>> 
>>>>>>> whose first 16-bit parcel is the same as c.li and whose second 16-bit
>>>>>>> parcel is ignored. I really do not think this is a good idea.
>>>>>>> 
>>>>>>>> Note that the Linux kernel needs the change pointed out in this patch,
>>>>>>>> to build w/o C.
>>>>>>> 
>>>>>>> That sounds like a Linux problem. Other OSes cope just fine. Fixing in
>>>>>>> firmware is not the right approach; fix your software instead. Which
>>>>>>> may mean abandoning an image format that isn’t fit for purpose. But the
>>>>>>> solution isn’t to hack in whatever random crud Linux wants in firmware,
>>>>>>> and other non-firmware SBI implementations like hypervisors.
>>>>>> 
>>>>>> There are other kernels that people care about?
>>>>>> 
>>>>>>> So this is a resounding NAK from me.
>>>>>> 
>>>>>> Jokes aside; Fair enough! I'll go back to the drawing board.
>>>>> 
>>>>> The kernel cannot handle this problem because the kernel has not initialized
>>>>> stvec at this time. And the MZ cannot be changed because it is part of the PE
>>>>> header. The best way is to skip the MZ of the PE header through the upper
>>>>> bootloader. Therefore, when the kernel is used as the payload of opensbi,
>>>>> skipped operations also need to be added. sbi_hart_switch_mode needs to add
>>>>> some code to detect whether the current core supports C extension, whether
>>>>> there is 0x5a4d5a4d at next_addr, and modify next_addr.
>>>> 
>>>> That’s also extremely ugly; now you’re just skipping instructions in
>>>> the payload rather than trapping like the current de-facto spec would
>>>> mandate. And what would you skip? Again, you can’t just skip 2 bytes,
>>>> you would have to skip a whole 4 bytes, which is more than just the
>>>> c.li instruction. And what happens if a vendor, not implementing C,
>>>> wants to use that encoding space? For example, Qualcomm, who are
>>>> proposing to ditch C entirely. It may not conform to a standard
>>>> profile, but it is a thing that SBI must support.
>>>> 
>>>> Linux is abusing file formats and trying to create a polyglot where
>>>> it’s not possible. That’s Linux’s problem, and it needs to stop
>>>> assuming things that aren’t true. This grotesque approach is not
>>>> present in other OSes.
>>> How do other operating systems solve it? This is a problem with the uefi
>>> image file format. Can we remove dos header?
>> 
>> They don’t delude themselves that you can just jump to the start of a
>> UEFI PE/COFF executable and interpret it as machine code.
> Show me code.

They are separate binaries, one for UEFI PE/COFF and one for direct SBI
booting if you want it. Unless you introduce a new image format for SBI
payloads there is no way around that without inappropriate hacks like
these suggestions. So there is no magic code. The answer is, like I’ve
been saying, to just not, because it’s not possible.

Jess