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

Andy Gross
2016-08-22T15:16:45Z

On Mon, Aug 22, 2016 at 03:53:26PM +0100, Will Deacon wrote:
> On Mon, Aug 22, 2016 at 09:02:46AM -0500, Andy Gross wrote:
> > On Mon, Aug 22, 2016 at 02:43:14PM +0100, Will Deacon wrote:
> > > On Sat, Aug 20, 2016 at 12:51:13AM -0500, Andy Gross wrote:
> > > > This patch adds the SMC Session ID to the results passed back from SMC
> > > > calls. The Qualcomm SMC implementation provides for interrupted SMC
> > > > functions. When this occurs, the SMC call will return a session ID that
> > > > is required to be used when resuming the interrupted SMC call.
> > > >
> > > > Signed-off-by: Andy Gross <andy.gross@linaro.org>
> > > > ---
> > > > arch/arm64/kernel/asm-offsets.c | 1 +
> > > > arch/arm64/kernel/smccc-call.S | 1 +
> > > > include/linux/arm-smccc.h | 4 +++-
> > > > 3 files changed, 5 insertions(+), 1 deletion(-)
> > >
> > > [...]
> > >
> > > > diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
> > > > index b5abfda..82d919f 100644
> > > > --- a/include/linux/arm-smccc.h
> > > > +++ b/include/linux/arm-smccc.h
> > > > @@ -63,18 +63,20 @@
> > > > /**
> > > > * struct arm_smccc_res - Result from SMC/HVC call
> > > > * @a0-a3 result values from registers 0 to 3
> > > > + * @a6 Session ID register (optional)
> > > > */
> > > > struct arm_smccc_res {
> > > > unsigned long a0;
> > > > unsigned long a1;
> > > > unsigned long a2;
> > > > unsigned long a3;
> > > > + unsigned long a6;
> > > > };
> > > >
> > > > /**
> > > > * arm_smccc_smc() - make SMC calls
> > > > * @a0-a7: arguments passed in registers 0 to 7
> > > > - * @res: result values from registers 0 to 3
> > > > + * @res: result values from registers 0 to 3 and optional register 6
> > >
> > > AFAICT from reading the SMCCC spec, parameter register 6 is "Unpredictable,
> > > Scratch registers" in return state, so I don't think this is correct.
> > >
> > > What am I missing?
> >
> > In the case of Qualcomm's implementation, they return a value in register 6 that
> > may or may not be used in subsequent calls. If I want to leverage the arm_smccc
> > functions, then I need to extend them to include the optional return value. The
> > downside to this is that everyone who uses this is exposed to it.
>
> Yes, I'm not keen on forcing this behaviour for everybody, as you never
> know what other firmware might do with unexpected a6 values. Could we
> perhaps quirk it, along the lines of the completely untested patch below?

A quirk would work fine. I'll try this out and get back to you.

Thanks,

Andy

> --->8
>
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 05070b72fc28..1895e87d0240 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -141,6 +141,8 @@ int main(void)
> #endif
> DEFINE(ARM_SMCCC_RES_X0_OFFS, offsetof(struct arm_smccc_res, a0));
> DEFINE(ARM_SMCCC_RES_X2_OFFS, offsetof(struct arm_smccc_res, a2));
> + DEFINE(ARM_SMCCC_RES_QUIRK_ID_OFFS, offsetof(struct arm_smccc_res, quirk.id));
> + DEFINE(ARM_SMCCC_RES_QUIRK_STATE_OFFS, offsetof(struct arm_smccc_res, quirk.state));
> BLANK();
> DEFINE(HIBERN_PBE_ORIG, offsetof(struct pbe, orig_address));
> DEFINE(HIBERN_PBE_ADDR, offsetof(struct pbe, address));
> diff --git a/arch/arm64/kernel/smccc-call.S b/arch/arm64/kernel/smccc-call.S
> index ae0496fa4235..3c6c976eaf5c 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 +21,12 @@
> ldr x4, [sp]
> stp x0, x1, [x4, #ARM_SMCCC_RES_X0_OFFS]
> stp x2, x3, [x4, #ARM_SMCCC_RES_X2_OFFS]
> - ret
> + ldr x9, [x4, #ARM_SMCCC_RES_QUIRK_ID_OFFS]
> + cbz x9, 1f /* ARM_SMCCC_QUIRK_NONE */
> + cmp x9, #ARM_SMCCC_QUIRK_QCOM_A6
> + b.ne 1f
> + str x6, [x4, ARM_SMCCC_RES_QUIRK_STATE_OFFS]
> +1: ret
> .cfi_endproc
> .endm
>
> diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
> index b5abfda80465..a3a6e291feb6 100644
> --- a/include/linux/arm-smccc.h
> +++ b/include/linux/arm-smccc.h
> @@ -14,9 +14,6 @@
> #ifndef __LINUX_ARM_SMCCC_H
> #define __LINUX_ARM_SMCCC_H
>
> -#include <linux/linkage.h>
> -#include <linux/types.h>
> -
> /*
> * This file provides common defines for ARM SMC Calling Convention as
> * specified in
> @@ -60,6 +57,21 @@
> #define ARM_SMCCC_OWNER_TRUSTED_OS 50
> #define ARM_SMCCC_OWNER_TRUSTED_OS_END 63
>
> +#define ARM_SMCCC_QUIRK_NONE 0
> +#define ARM_SMCCC_QUIRK_QCOM_A6 1 /* Save/restore register a6 */
> +
> +#ifndef __ASSEMBLY__
> +
> +#include <linux/linkage.h>
> +#include <linux/types.h>
> +
> +struct arm_smccc_quirk {
> + int id;
> + union {
> + unsigned long a6;
> + } state;
> +};
> +
> /**
> * struct arm_smccc_res - Result from SMC/HVC call
> * @a0-a3 result values from registers 0 to 3
> @@ -69,6 +81,7 @@ struct arm_smccc_res {
> unsigned long a1;
> unsigned long a2;
> unsigned long a3;
> + struct arm_smccc_quirk quirk;
> };
>
> /**
> @@ -101,4 +114,5 @@ asmlinkage void arm_smccc_hvc(unsigned long a0, unsigned long a1,
> unsigned long a5, unsigned long a6, unsigned long a7,
> struct arm_smccc_res *res);
>
> +#endif /* !__ASSEMBLY__ */
> #endif /*__LINUX_ARM_SMCCC_H*/
> --
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