Subject: Re: [PATCH v7 1/7] powerpc/pmem: Restrict papr_scm to P8 and above.

From: Michal Suchánek <[email-masked]>
Date: Fri, 21 Jan 2022 09:40:56 +0100

Hello,

On Wed, Jul 01, 2020 at 12:52:29PM +0530, Aneesh Kumar K.V wrote:
> The PAPR based virtualized persistent memory devices are only supported on
> POWER9 and above. In the followup patch, the kernel will switch the persistent
> memory cache flush functions to use a new `dcbf` variant instruction. The new
> instructions even though added in ISA 3.1 works even on P8 and P9 because these
> are implemented as a variant of existing `dcbf` and `hwsync` and on P8 and
> P9 behaves as such.
> 
> Considering these devices are only supported on P8 and above,  update the driver
> to prevent a P7-compat guest from using persistent memory devices.
> 
> We don't update of_pmem driver with the same condition, because, on bare-metal,
> the firmware enables pmem support only on P9 and above. There the kernel depends
> on OPAL firmware to restrict exposing persistent memory related device tree
> entries on older hardware. of_pmem.ko is written without any arch dependency and
> we don't want to add ppc64 specific cpu feature check in of_pmem driver.
> 
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/platforms/pseries/pmem.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/platforms/pseries/pmem.c b/arch/powerpc/platforms/pseries/pmem.c
> index f860a897a9e0..2347e1038f58 100644
> --- a/arch/powerpc/platforms/pseries/pmem.c
> +++ b/arch/powerpc/platforms/pseries/pmem.c
> @@ -147,6 +147,12 @@ const struct of_device_id drc_pmem_match[] = {
>  
>  static int pseries_pmem_init(void)
>  {
> +	/*
> +	 * Only supported on POWER8 and above.
> +	 */
> +	if (!cpu_has_feature(CPU_FTR_ARCH_207S))
> +		return 0;
> +

This looks superfluous.

The hypervisor is responsible for publishing the pmem in devicetree when
present, kernel is responsible for using it when supported by the
kernel.

Or is there a problem that the flush instruction is not available in P7
compat mode?

Even then volatile regions should still work.

Thanks

Michal