Re: [PATCH 2/5] x86/sgx: Add kernel-doc descriptions for params passed to vDSO user handler

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 13, 2025, Kai Huang wrote:
> On Wed, 2025-11-12 at 08:07 -0800, Sean Christopherson wrote:
> > Add kernel-doc markup for the register parameters passed by the vDSO blob
> > to the user handler to suppress build warnings, e.g.
> > 
> >   WARNING: arch/x86/include/uapi/asm/sgx.h:157 function parameter 'r8' not
> >            described in 'sgx_enclave_user_handler_t'
> > 
> > Call out that except for RSP, the registers are undefined on asynchronous
> > exits as far as the vDSO ABI is concerned.  E.g. the vDSO's exception
> > handler clobbers RDX, RDI, and RSI, and the kernel doesn't guarantee that
> > R8 or R9 will be zero (the synthetic value loaded by the CPU).
> 
> Perhaps a nit:
> 
> I am not that familiar with this part, but AFAICT the kernel always sets
> RDI/RSI/RDX to exception vector/error code/addr before invoking the user
> handler, after the vDSO's exception handler clobbers them.
> 
> Since you are adding description to them, should we somehow call this out in
> the comment, if I didn't miss anything?

I don't think we want to commit to that as ABI for the vDSO interface, which is
why I documented the values as being "undefined".  Maybe were already stuck with
that ABI, e.g. if some funky userspace is looking at the register params instead
of its run structure, but IMO we should at least discourage relying on the values.

E.g. if the kernel ever changed its exception fixup to use different registers,
then RDI/RSI/RDX might not hold the same values.  In hindsight, we probably should
have scrambled those registers after propagating the exception information to the
run structure.