Re: [PATCH 1/2] arm64: kernel: Add SMC Session ID to results

Andy Gross
2016-08-23T12:36:22Z

On Tue, Aug 23, 2016 at 11:38:41AM +0100, Lorenzo Pieralisi wrote:
> On Mon, Aug 22, 2016 at 05:38:31PM -0700, Stephen Boyd wrote:
>
> [...]
>
> > This all comes about because the firmware generates a session id
> > for the SMC call and jams it in x6. The assembly on the
> > non-secure side is written with a tight loop around the smc
> > instruction so that when the return value indicates
> > "interrupted", x6 is kept intact and the non-secure OS can jump
> > back to the secure OS without register reloading. Perhaps
> > referring to x6 as result value is not correct because it's
> > really a session id that's irrelevant once the smc call
> > completes.
>
> Sorry I missed this bit. The session id is _generated_ by secure
> firmware (probably only when the value passed in x6 == 0 (?))
> and actually returned to the caller so that subsequent (interrupted)
> calls can re-issue the same value, is that correct ?

Yes, that is exactly what is going on. You always pass in 0 for the first call.
If the call is interrupted and needs to be re-executed, you will get a specific
result in a0 that tells you to redo the call using x6 as your session ID.

>
> If that's the case the value in x6 is a result value from an SMCCC
> perspective and your current FW is not SMCCC compliant.

Should we then write our own ASM snippet to do exactly what we want? It'd be
the same as the arm_smccc except with the extra str. I'm ok with that, I was
just hoping to leverage the existing smccc code. The quirk also works well,
except it costs everyone else 1 load and compare.

Regards,

Andy