Auto-enabling V unit and/or use of elf attributes (was Re: Adding V-ext regs to signal context w/o expanding kernel struct sigcontext to avoid glibc ABI break)

[Anup Patel]

On Wed, Jan 11, 2023 at 6:53 AM Richard Henderson
<richard.henderson at linaro.org> wrote:
>
> On 1/10/23 10:07, Vineet Gupta wrote:
> > Yes bulk of glibc might not have vector code, but those V ifunc routines do and IMO this
> > information needs to be recorded somewhere in the elf. Case in point being the current
> > issue with how to enable V unit. Community wants a per-process enable, using an explicit
> > prctl from userspace (since RV doesn't have fault-on-first use hardware mechanism unlike
> > some of the other arches). But how does the glibc loader know to invoke prctl. We can't
> > just rely on user env GLIBC_TUNABLE etc since that might not be accurate. It needs
> > somethign concrete which IMO can come from elf attributes. If not, do you have suggestions
> > on how to solve this issue ?
>
> Why not just fault on first use to enable?  That's vastly less complicated than trying to
> plumb anything through elf resulting in a prctl.
>
> You might say "but the fault could fail to allocate memory", but honestly, the prctl isn't
> able to fail either -- if it doesn't work, the process must exit.  And, surely, there's
> some minimal vector configuration for which the allocation must succeed.

IMO, this is a very good suggestion.

For the benefit of everyone, both sstatus.FS and sstatus.VS have the
following states:
1. Off (0): All off and any access to float / vector will result in exception
2. Initial (1): None dirty or clean, some on
3. Clean (2): None dirty, some clean
4. Dirty (3): Some dirty

For float, we are setting sstatus.FS = 1 (Initial) in start_thread() by
default for all tasks and we are doing lazy save-restore in fstate_save()
and fstate_restore().

For vector, we can take a different approach where start_thread()
will by default set sstatus.VS = 0 (Off) for all tasks. Now whenever
any task access vector state, Linux RISC-V will get an exception
and at that point in time we can allocate memory for the vector
state and also set sstatus.VS = 1 (Initial) for that task. The save
restore of the vector state can still be lazy for the tasks using it.

Regards,
Anup