[v2,12/21] arm64/sme: Implement ZT0 ptrace support

Implement support for a new note type NT_ARM64_ZT providing access to
ZT0 when implemented. Since ZT0 is a register with constant size this is
much simpler than for other SME state.

As ZT0 is only accessible when PSTATE.ZA is set writes to ZT0 cause
PSTATE.ZA to be set, the main alternative would be to return -EBUSY in
this case but this seemed more constructive. Practical users are also
going to be working with ZA anyway and have some understanding of the
state.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kernel/ptrace.c | 54 ++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/elf.h   |  1 +
 2 files changed, 55 insertions(+)

On 11/1/22 14:33, Mark Brown wrote:
> Implement support for a new note type NT_ARM64_ZT providing access to

Should we use NT_AARCH64_ZT instead? This would align with our previous choice of PT_AARCH64_MEMTAG_MTE.

And we have a precedent with PT_AARCH64_ARCHEXT as well.

> ZT0 when implemented. Since ZT0 is a register with constant size this is
> much simpler than for other SME state.
> 
> As ZT0 is only accessible when PSTATE.ZA is set writes to ZT0 cause
> PSTATE.ZA to be set, the main alternative would be to return -EBUSY in
> this case but this seemed more constructive. Practical users are also
> going to be working with ZA anyway and have some understanding of the
> state.
> 
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>   arch/arm64/kernel/ptrace.c | 54 ++++++++++++++++++++++++++++++++++++++
>   include/uapi/linux/elf.h   |  1 +
>   2 files changed, 55 insertions(+)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 85105375bea5..a508f3a69d9f 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1145,6 +1145,51 @@ static int za_set(struct task_struct *target,
>   	return ret;
>   }
>   
> +static int zt_get(struct task_struct *target,
> +		  const struct user_regset *regset,
> +		  struct membuf to)
> +{
> +	if (!system_supports_sme2())
> +		return -EINVAL;
> +
> +	/*
> +	 * If PSTATE.ZA is not set then ZT will be zeroed when it is
> +	 * enabled so report the current register value as zero.
> +	 */
> +	if (thread_za_enabled(&target->thread))
> +		membuf_write(&to, thread_zt_state(&target->thread),
> +			     ZT_SIG_REG_BYTES);
> +	else
> +		membuf_zero(&to, ZT_SIG_REG_BYTES);
> +
> +	return 0;
> +}
> +
> +static int zt_set(struct task_struct *target,
> +		  const struct user_regset *regset,
> +		  unsigned int pos, unsigned int count,
> +		  const void *kbuf, const void __user *ubuf)
> +{
> +	int ret;
> +
> +	if (!system_supports_sme2())
> +		return -EINVAL;
> +
> +	if (!thread_za_enabled(&target->thread)) {
> +		sme_alloc(target);
> +		if (!target->thread.sme_state)
> +			return -ENOMEM;
> +	}
> +
> +	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
> +				 thread_zt_state(&target->thread),
> +				 0, ZT_SIG_REG_BYTES);
> +	if (ret == 0)
> +		target->thread.svcr |= SVCR_ZA_MASK;
> +
> +	return ret;
> +}
> +
>   #endif /* CONFIG_ARM64_SME */
>   
>   #ifdef CONFIG_ARM64_PTR_AUTH
> @@ -1367,6 +1412,7 @@ enum aarch64_regset {
>   #ifdef CONFIG_ARM64_SVE
>   	REGSET_SSVE,
>   	REGSET_ZA,
> +	REGSET_ZT,
>   #endif
>   #ifdef CONFIG_ARM64_PTR_AUTH
>   	REGSET_PAC_MASK,
> @@ -1474,6 +1520,14 @@ static const struct user_regset aarch64_regsets[] = {
>   		.regset_get = za_get,
>   		.set = za_set,
>   	},
> +	[REGSET_ZT] = { /* SME ZA */
> +		.core_note_type = NT_ARM_ZT,
> +		.n = 1,
> +		.size = ZT_SIG_REG_BYTES,
> +		.align = sizeof(u64),
> +		.regset_get = zt_get,
> +		.set = zt_set,
> +	},
>   #endif
>   #ifdef CONFIG_ARM64_PTR_AUTH
>   	[REGSET_PAC_MASK] = {
> diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
> index c7b056af9ef0..3a73c853c537 100644
> --- a/include/uapi/linux/elf.h
> +++ b/include/uapi/linux/elf.h
> @@ -434,6 +434,7 @@ typedef struct elf64_shdr {
>   #define NT_ARM_PAC_ENABLED_KEYS	0x40a	/* arm64 ptr auth enabled keys (prctl()) */
>   #define NT_ARM_SSVE	0x40b		/* ARM Streaming SVE registers */
>   #define NT_ARM_ZA	0x40c		/* ARM SME ZA registers */
> +#define NT_ARM_ZT	0x40d		/* ARM SME ZT registers */

I guess the above comment means this would also need to be NT_AARCH64_ZT, but historically we've been using NT_ARM_* even for AARCH64.

So I suppose this is OK.

>   #define NT_ARC_V2	0x600		/* ARCv2 accumulator/extra registers */
>   #define NT_VMCOREDD	0x700		/* Vmcore Device Dump Note */
>   #define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers */

On Fri, Nov 11, 2022 at 10:31:00AM +0000, Luis Machado wrote:
> On 11/1/22 14:33, Mark Brown wrote:

> > Implement support for a new note type NT_ARM64_ZT providing access to

> Should we use NT_AARCH64_ZT instead? This would align with our previous choice of PT_AARCH64_MEMTAG_MTE.

> And we have a precedent with PT_AARCH64_ARCHEXT as well.

> >   #define NT_ARM_ZA	0x40c		/* ARM SME ZA registers */
> > +#define NT_ARM_ZT	0x40d		/* ARM SME ZT registers */

> I guess the above comment means this would also need to be NT_AARCH64_ZT, but historically we've been using NT_ARM_* even for AARCH64.

> So I suppose this is OK.

Indeed - there's also a lot of NT_ARM_ defines, including all the other
FP defines (NT_ARM_SVE, NT_ARM_SSVE and NT_ARM_ZA) and PAC.  It feels
better to be consistent with at least the other FP stuff.  It's only the
MTE define and the .note.gnu.property stuff that's used AARCH64, and
things like SVE are fairly old.