linux-kernel - Re: [Patch v4 2/2] firmware: qcom: scm: Fix interrupted SCM calls

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Date:   Mon, 30 Jan 2017 22:22:18 -0800
From:   Olof Johansson <olof@...om.net>
To:     Will Deacon <will.deacon@....com>
Cc:     Andy Gross <andy.gross@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Kevin Hilman <khilman@...libre.com>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [Patch v4 2/2] firmware: qcom: scm: Fix interrupted SCM calls

On Mon, Jan 30, 2017 at 2:55 AM, Will Deacon <will.deacon@....com> wrote:
> Hi Olof,
>
> On Sun, Jan 29, 2017 at 04:24:51PM -0800, Olof Johansson wrote:
>> On Thu, Jan 19, 2017 at 8:58 AM, Andy Gross <andy.gross@...aro.org> wrote:
>> > This patch adds a Qualcomm specific quirk to the arm_smccc_smc call.
>> >
>> > On Qualcomm ARM64 platforms, the SMC call can return before it has
>> > completed.  If this occurs, the call can be restarted, but it requires
>> > using the returned session ID value from the interrupted SMC call.
>> >
>> > The quirk stores off the session ID from the interrupted call in the
>> > quirk structure so that it can be used by the caller.
>> >
>> > This patch folds in a fix given by Sricharan R:
>> > https://lkml.org/lkml/2016/9/28/272
>> >
>> > Signed-off-by: Andy Gross <andy.gross@...aro.org>
>> > Reviewed-by: Will Deacon <will.deacon@....com>
>> > ---
>> >  arch/arm64/kernel/smccc-call.S |  9 ++++++++-
>> >  drivers/firmware/qcom_scm-64.c | 13 ++++++++++---
>> >  include/linux/arm-smccc.h      | 11 ++++++++---
>> >  3 files changed, 26 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/arch/arm64/kernel/smccc-call.S b/arch/arm64/kernel/smccc-call.S
>> > index 6290696..72ecdca 100644
>> > --- a/arch/arm64/kernel/smccc-call.S
>> > +++ b/arch/arm64/kernel/smccc-call.S
>> > @@ -12,6 +12,7 @@
>> >   *
>> >   */
>> >  #include <linux/linkage.h>
>> > +#include <linux/arm-smccc.h>
>> >  #include <asm/asm-offsets.h>
>> >
>> >         .macro SMCCC instr
>> > @@ -20,7 +21,13 @@
>> >         ldr     x4, [sp]
>> >         stp     x0, x1, [x4, #ARM_SMCCC_RES_X0_OFFS]
>> >         stp     x2, x3, [x4, #ARM_SMCCC_RES_X2_OFFS]
>> > -       ret
>> > +       ldr     x4, [sp, #8]
>> > +       cbz     x4, 1f /* no quirk structure */
>> > +       ldr     x9, [x4, #ARM_SMCCC_QUIRK_ID_OFFS]
>> > +       cmp     x9, #ARM_SMCCC_QUIRK_QCOM_A6
>> > +       b.ne    1f
>> > +       str     x6, [x4, ARM_SMCCC_QUIRK_STATE_OFFS]
>> > +1:     ret
>> >         .cfi_endproc
>> >         .endm
>>
>> This extends the SMC entry/return path quite a bit.
>
> I honestly doubt it's measurable. You've got an independent load from the
> stack and a cbz that's likely predicted correctly given the static nature
> of the quirk. Then you have an SMC, which is going to trap and dominate
> the cost of this function.
>
>> Is this truly a qualcomm-only quirk, or are other vendors also picking it
>> up?
>
> Currently, it's just qualcomm. Whilst I'd love to say they'll be the only
> people to interpret the SMCCC in an imaginative fashion, I'd be surprised
> if we don't see other vendors making mistakes in this area in the future.

Ok, so the list of checks is anticipated to grow.

>> Why not either make arm_smccc_.* function pointers and update them
>> accordingly, or use a custom version for the specific locations where
>> you want/need to restart the calls? You are after all already wrapping
>> them in qcom_scm_call().
>
> Having the low-level SMC entry code in one place is advantageous because
> it means the SMCCC contract is enforced in common code, making it easier
> to debug and maintain. If a vendor got the contract so badly wrong that
> it didn't resemble SMCCC, then I'd agree with you, but here we're just
> saving and restoring an extra register.

What contract? Qualcomm just violated it and the answer isn't to
enforce, it's to enable their "enhanced" implementation (and it should
be, within reason).

It's not like their own special SMCCC functions have to go in a
different file. Stick them on the side of the current ones in the same
file. The main call is already nicely abstracted with an asm macro so
that part will be shared, and the call sites are per-vendor anyway.

>> Seems like a more appropriate change than burden all platforms with
>> longer code path due to your quirk.
>
> I really don't think it's a problem. Do you have numbers suggesting
> otherwise?

Not on this first quirk, no.

Anyway, I guess I'm just bikeshedding. You should merge this code if
you're happy with it.



Thanks!

-Olof