Subject: Re: [PATCH v3 1/3] powerpc/mm: Enable radix GTSE only if supported.

From: Bharata B Rao <[email-masked]>
Date: Mon, 6 Jul 2020 09:47:46 +0530

On Mon, Jul 06, 2020 at 07:19:02AM +0530, Santosh Sivaraj wrote:
> 
> Hi Bharata,
> 
> Bharata B Rao <[email-masked]> writes:
> 
> > Make GTSE an MMU feature and enable it by default for radix.
> > However for guest, conditionally enable it if hypervisor supports
> > it via OV5 vector. Let prom_init ask for radix GTSE only if the
> > support exists.
> >
> > Having GTSE as an MMU feature will make it easy to enable radix
> > without GTSE. Currently radix assumes GTSE is enabled by default.
> >
> > Signed-off-by: Bharata B Rao <[email-masked]>
> > Reviewed-by: Aneesh Kumar K.V <[email-masked]>
> > ---
> >  arch/powerpc/include/asm/mmu.h    |  4 ++++
> >  arch/powerpc/kernel/dt_cpu_ftrs.c |  1 +
> >  arch/powerpc/kernel/prom_init.c   | 13 ++++++++-----
> >  arch/powerpc/mm/init_64.c         |  5 ++++-
> >  4 files changed, 17 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
> > index f4ac25d4df05..884d51995934 100644
> > --- a/arch/powerpc/include/asm/mmu.h
> > +++ b/arch/powerpc/include/asm/mmu.h
> > @@ -28,6 +28,9 @@
> >   * Individual features below.
> >   */
> >  
> > +/* Guest Translation Shootdown Enable */
> > +#define MMU_FTR_GTSE			ASM_CONST(0x00001000)
> > +
> >  /*
> >   * Support for 68 bit VA space. We added that from ISA 2.05
> >   */
> > @@ -173,6 +176,7 @@ enum {
> >  #endif
> >  #ifdef CONFIG_PPC_RADIX_MMU
> >  		MMU_FTR_TYPE_RADIX |
> > +		MMU_FTR_GTSE |
> >  #ifdef CONFIG_PPC_KUAP
> >  		MMU_FTR_RADIX_KUAP |
> >  #endif /* CONFIG_PPC_KUAP */
> > diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
> > index a0edeb391e3e..ac650c233cd9 100644
> > --- a/arch/powerpc/kernel/dt_cpu_ftrs.c
> > +++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
> > @@ -336,6 +336,7 @@ static int __init feat_enable_mmu_radix(struct dt_cpu_feature *f)
> >  #ifdef CONFIG_PPC_RADIX_MMU
> >  	cur_cpu_spec->mmu_features |= MMU_FTR_TYPE_RADIX;
> >  	cur_cpu_spec->mmu_features |= MMU_FTRS_HASH_BASE;
> > +	cur_cpu_spec->mmu_features |= MMU_FTR_GTSE;
> >  	cur_cpu_spec->cpu_user_features |= PPC_FEATURE_HAS_MMU;
> >  
> >  	return 1;
> > diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
> > index 90c604d00b7d..cbc605cfdec0 100644
> > --- a/arch/powerpc/kernel/prom_init.c
> > +++ b/arch/powerpc/kernel/prom_init.c
> > @@ -1336,12 +1336,15 @@ static void __init prom_check_platform_support(void)
> >  		}
> >  	}
> >  
> > -	if (supported.radix_mmu && supported.radix_gtse &&
> > -	    IS_ENABLED(CONFIG_PPC_RADIX_MMU)) {
> > -		/* Radix preferred - but we require GTSE for now */
> > -		prom_debug("Asking for radix with GTSE\n");
> > +	if (supported.radix_mmu && IS_ENABLED(CONFIG_PPC_RADIX_MMU)) {
> > +		/* Radix preferred - Check if GTSE is also supported */
> > +		prom_debug("Asking for radix\n");
> >  		ibm_architecture_vec.vec5.mmu = OV5_FEAT(OV5_MMU_RADIX);
> > -		ibm_architecture_vec.vec5.radix_ext = OV5_FEAT(OV5_RADIX_GTSE);
> > +		if (supported.radix_gtse)
> > +			ibm_architecture_vec.vec5.radix_ext =
> > +					OV5_FEAT(OV5_RADIX_GTSE);
> > +		else
> > +			prom_debug("Radix GTSE isn't supported\n");
> >  	} else if (supported.hash_mmu) {
> >  		/* Default to hash mmu (if we can) */
> >  		prom_debug("Asking for hash\n");
> > diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> > index bc73abf0bc25..152aa0200cef 100644
> > --- a/arch/powerpc/mm/init_64.c
> > +++ b/arch/powerpc/mm/init_64.c
> > @@ -407,12 +407,15 @@ static void __init early_check_vec5(void)
> >  		if (!(vec5[OV5_INDX(OV5_RADIX_GTSE)] &
> >  						OV5_FEAT(OV5_RADIX_GTSE))) {
> >  			pr_warn("WARNING: Hypervisor doesn't support RADIX with GTSE\n");
> > -		}
> > +			cur_cpu_spec->mmu_features &= ~MMU_FTR_GTSE;
> > +		} else
> > +			cur_cpu_spec->mmu_features |= MMU_FTR_GTSE;
> 
> The GTSE flag is set by default in feat_enable_mmu_radix(), should it
> be set again here?

Strictly speaking no, but makes it a bit explicit and also follows what
the related feature does below:

> >  		/* Do radix anyway - the hypervisor said we had to */
> >  		cur_cpu_spec->mmu_features |= MMU_FTR_TYPE_RADIX;

Regards,
Bharata.