Re: [PATCH RFC 1/2] powerpc/pseries: papr-vpd char driver for VPD retrieval

[Michael Ellerman <mpe@ellerman.id.au>]

Michal Suchánek <msuchanek@suse.de> writes:
> On Thu, Aug 31, 2023 at 12:59:25PM -0500, Nathan Lynch wrote:
...
>> You (Michal) seem to favor a kernel-user ABI where user space is allowed
>> to invoke arbitrary RTAS functions by name. But we already have that in
>> the form of the rtas() syscall. (User space looks up function tokens by
>> name in the DT.) The point of the series is that we need to move away
>> from that. It's too low-level and user space has to use /dev/mem when
>> invoking any of the less-simple RTAS functions.
>
> We don't have that, directly accessing /dev/mem does not really work.
> And that's what needs fixing in my view.
>
> The rtas calls are all mechanically the same, the function implemented
> here should be able to call any of them if there was a way to specify
> the call.
>
> Given that there is desire to have access to multiple calls I don't
> think it makes sense to allocate a separate device with different name
> for each.

I think it does make sense.

We explicitly don't want a general "call any RTAS function" API.

We want tightly scoped APIs that do one thing, or a family of related
things, but not anything & everything.

Having different devices for each of those APIs means permissions can be
granted separately on those devices. So a user/group can be given access
to the "papr-vpd" device, but not some other unrelated device that also
happens to expose an RTAS service (eg. error injection).

cheers