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

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  Re: [PATCH 1/2] arm64: kernel: Add SMC Session ID to results Andy Gross <andy.gross@linaro.org> - 2016-08-30 22:20 +0200
    Re: [PATCH 1/2] arm64: kernel: Add SMC Session ID to results Will Deacon <will.deacon@arm.com> - 2016-08-31 16:40 +0200

#1472839 — Re: [PATCH 1/2] arm64: kernel: Add SMC Session ID to results

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 ?
> 
> 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.
> 

So is Will's solution to this ok?  If so I will respin with the minor change to
get it working and resend.  If not, do I roll my own smccc wrapper?

Regards,

Andy

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#1473552

On Tue, Aug 30, 2016 at 03:16:42PM -0500, Andy Gross wrote:
> 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 ?
> > 
> > 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.
> > 
> 
> So is Will's solution to this ok?  If so I will respin with the minor change to
> get it working and resend.  If not, do I roll my own smccc wrapper?

Obviously I'm biased, but I prefer to handle this as a quirk to make it
clear that it's a vendor-specific extension to the SMCCC, so if you
could post a patch based on the diff I sent, that would be great.

You'll also need to:

  (1) Make sure you don't break 32-bit ARM
  (2) Make sure that struct arm_smccc_res is always zero-initialised by
      its other users (to ensure that QUIRK_NONE is set). In fact, it
      might be nicer to pass the quirk structure as a separate argument,
      rather than embed it in arm_smccc_res.

Will

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