[RFC PATCH 06/13] powerpc/dexcr: Add prctl implementation

[Nicholas Piggin]

On Mon Nov 28, 2022 at 12:44 PM AEST, Benjamin Gray wrote:
> Adds an initial prctl interface implementation. Unprivileged processes
> can query the current prctl setting, including whether an aspect is
> implemented by the hardware or is permitted to be modified by a setter
> prctl. Editable aspects can be changed by a CAP_SYS_ADMIN privileged
> process.
>
> The prctl setting represents what the process itself has requested, and
> does not account for any overrides. Either the kernel or a hypervisor
> may enforce a different setting for an aspect.
>
> Userspace can access a readonly view of the current DEXCR via SPR 812,
> and a readonly view of the aspects enforced by the hypervisor via
> SPR 455. A bitwise OR of these two SPRs will give the effective
> DEXCR aspect state of the process.

You said (offline) that you were looking at the PR_SPEC_* speculation
control APIs but that this was different enough that you needed a
different one.

It would be good to know what some of those issues were in the
changelog, would be nice to have some docs (could we add something
to spec_ctrl.rst maybe?). I assume at least one difference is that
some of our bits are not speculative but architectural (e.g., the
stack hash check).

I also wonder if we could implement some of the PR_SPEC controls
APIs by mapping relevant DEXCR aspects to them instead of (or as well
as) the DEXCR controls? Or would the PR_SPEC users be amenable to
extensions that make our usage fit a bit better?

I'm just thinking if we can reduce reliance on arch specific APIs a
bit would be nice.

>
> Signed-off-by: Benjamin Gray <bgray at linux.ibm.com>
> ---
>  arch/powerpc/include/asm/processor.h |  13 +++
>  arch/powerpc/kernel/dexcr.c          | 133 ++++++++++++++++++++++++++-
>  arch/powerpc/kernel/process.c        |   6 ++
>  3 files changed, 151 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
> index 2381217c95dc..4c995258f668 100644
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -265,6 +265,9 @@ struct thread_struct {
>  	unsigned long   sier2;
>  	unsigned long   sier3;
>  	unsigned long	hashkeyr;
> +	unsigned int	dexcr_override;
> +	unsigned int	dexcr_mask;

Hmm, what's the mask doing here? It only gets bits set and never
cleared AFAIKS. What is different between an initial state and a
SET then CLEAR state?

Thanks,
Nick