Re: [PATCH 0/5] Add parsing for Zimop ISA extension

[Clément Léger]

On 08/04/2024 13:03, Andrew Jones wrote:
> On Mon, Apr 08, 2024 at 10:01:12AM +0200, Clément Léger wrote:
>>
>>
>> On 05/04/2024 19:33, Deepak Gupta wrote:
>>> On Fri, Apr 5, 2024 at 8:26 AM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> On Thu, Apr 04, 2024 at 12:32:46PM +0200, Clément Léger wrote:
>>>>> The Zimop ISA extension was ratified recently. This series adds support
>>>>> for parsing it from riscv,isa, hwprobe export and kvm support for
>>>>> Guest/VM.
>>>>
>>>> I'm not sure we need this. Zimop by itself isn't useful, so I don't know
>>>> if we need to advertise it at all. When an extension comes along that
>>>> redefines some MOPs, then we'll advertise that extension, but the fact
>>>> Zimop is used for that extension is really just an implementation detail.
>>>
>>> Only situation I see this can be useful is this:--
>>>
>>> An implementer, implemented Zimops in CPU solely for the purpose that they can
>>> run mainline distro & packages on their hardware and don't want to leverage any
>>> feature which are built on top of Zimop.
>>
>> Yes, the rationale was that some binaries using extensions that overload
>> MOPs could still be run. With Zimop exposed, the loader could determine
>> if the binary can be executed without potentially crashing. We could
>> also let the program run anyway but the execution could potentially
>> crash unexpectedly, which IMHO is not really good for the user
>> experience nor for debugging. I already think that the segfaults which
>> happens when executing binaries that need some missing extension are not
>> so easy to debug, so better add more guards.
> 
> OK. It's only one more extension out of dozens, so I won't complain more,

No worries, your point *is* valid since I'm not sure yet that the loader
will actually do that one day.

BTW, are you aware of any effort to make the elf dynamic loader
"smarter" and actually check for needed extensions to be present rather
than blindly running the elf and potentially catching SIGILL ?

Thanks,

Clément

> but I was thinking that binaries that use particular extensions would
> check for those particular extensions (step 2), rather than Zimop.
> 
> Thanks,
> drew
> 
>>
>>>
>>> As an example zicfilp and zicfiss are dependent on zimops. glibc can
>>> do following
>>>
>>> 1) check elf header if binary was compiled with zicfiss and zicfilp,
>>> if yes goto step 2, else goto step 6.
>>> 2) check if zicfiss/zicfilp is available in hw via hwprobe, if yes
>>> goto step 5. else goto step 3
>>> 3) check if zimop is available via hwprobe, if yes goto step 6, else goto step 4
>>
>> I think you meant step 5 rather than step 6.
>>
>> Clément
>>
>>> 4) This binary won't be able to run successfully on this platform,
>>> issue exit syscall. <-- termination
>>> 5) issue prctl to enable shadow stack and landing pad for current task
>>> <-- enable feature
>>> 6) let the binary run <-- let the binary run because no harm can be done