Subject: Re: [PATCH v2 07/10] powerpc/perf: support BHRB disable bit and new filtering modes

From: Gautham R Shenoy <[email-masked]>
Date: Wed, 8 Jul 2020 13:13:15 +0530

On Tue, Jul 07, 2020 at 05:17:55PM +1000, Michael Neuling wrote:
> On Wed, 2020-07-01 at 05:20 -0400, Athira Rajeev wrote:
> > PowerISA v3.1 has few updates for the Branch History Rolling Buffer(BHRB).
> > First is the addition of BHRB disable bit and second new filtering
> > modes for BHRB.
> > 
> > BHRB disable is controlled via Monitor Mode Control Register A (MMCRA)
> > bit 26, namely "BHRB Recording Disable (BHRBRD)". This field controls
> > whether BHRB entries are written when BHRB recording is enabled by other
> > bits. Patch implements support for this BHRB disable bit.
> 
> Probably good to note here that this is backwards compatible. So if you have a
> kernel that doesn't know about this bit, it'll clear it and hence you still get
> BHRB. 
> 
> You should also note why you'd want to do disable this (ie. the core will run
> faster).
> 
> > Secondly PowerISA v3.1 introduce filtering support for
> > PERF_SAMPLE_BRANCH_IND_CALL/COND. The patch adds BHRB filter support
> > for "ind_call" and "cond" in power10_bhrb_filter_map().
> > 
> > 'commit bb19af816025 ("powerpc/perf: Prevent kernel address leak to userspace
> > via BHRB buffer")'
> > added a check in bhrb_read() to filter the kernel address from BHRB buffer.
> > Patch here modified
> > it to avoid that check for PowerISA v3.1 based processors, since PowerISA v3.1
> > allows
> > only MSR[PR]=1 address to be written to BHRB buffer.
> > 
> > Signed-off-by: Athira Rajeev <[email-masked]>
> > ---
> >  arch/powerpc/perf/core-book3s.c       | 27 +++++++++++++++++++++------
> >  arch/powerpc/perf/isa207-common.c     | 13 +++++++++++++
> >  arch/powerpc/perf/power10-pmu.c       | 13 +++++++++++--
> >  arch/powerpc/platforms/powernv/idle.c | 14 ++++++++++++++
> 
> This touches the idle code so we should get those guys on CC (adding Vaidy and
> Ego).
> 
> >  4 files changed, 59 insertions(+), 8 deletions(-)
> > 

[..snip..]

> > diff --git a/arch/powerpc/platforms/powernv/idle.c
> > b/arch/powerpc/platforms/powernv/idle.c
> > index 2dd4673..7db99c7 100644
> > --- a/arch/powerpc/platforms/powernv/idle.c
> > +++ b/arch/powerpc/platforms/powernv/idle.c
> > @@ -611,6 +611,7 @@ static unsigned long power9_idle_stop(unsigned long psscr,
> > bool mmu_on)
> >  	unsigned long srr1;
> >  	unsigned long pls;
> >  	unsigned long mmcr0 = 0;
> > +	unsigned long mmcra_bhrb = 0;

We are saving the whole of MMCRA aren't we ? We might want to just
name it mmcra in that case.

> >  	struct p9_sprs sprs = {}; /* avoid false used-uninitialised */
> >  	bool sprs_saved = false;
> >  
> > @@ -657,6 +658,15 @@ static unsigned long power9_idle_stop(unsigned long
> > psscr, bool mmu_on)
> >  		  */
> >  		mmcr0		= mfspr(SPRN_MMCR0);
> >  	}
> > +
> > +	if (cpu_has_feature(CPU_FTR_ARCH_31)) {
> > +		/* POWER10 uses MMCRA[:26] as BHRB disable bit
> > +		 * to disable BHRB logic when not used. Hence Save and
> > +		 * restore MMCRA after a state-loss idle.
> > +		 */

Multi-line comment usually has the first line blank.

		/*
	         * Line 1
		 * Line 2
		 * .
		 * .
		 * .
		 * Line N
		 */

> > +		mmcra_bhrb		= mfspr(SPRN_MMCRA);
> 
> 
> Why is the bhrb bit of mmcra special here?

The comment above could include the consequence of not saving and
restoring MMCRA i.e

- If the user hasn't asked for the BHRB to be
  written the value of MMCRA[BHRBD] = 1.

- On wakeup from stop, MMCRA[BHRBD] will be 0, since MMCRA is not a
  previleged resource and will be lost.

- Thus, if we do not save and restore the MMCRA[BHRBD], the hardware
  will be needlessly writing to the BHRB in the problem mode.

> 
> > +	}
> > +
> >  	if ((psscr & PSSCR_RL_MASK) >= pnv_first_spr_loss_level) {
> >  		sprs.lpcr	= mfspr(SPRN_LPCR);
> >  		sprs.hfscr	= mfspr(SPRN_HFSCR);
> > @@ -721,6 +731,10 @@ static unsigned long power9_idle_stop(unsigned long
> > psscr, bool mmu_on)
> >  			mtspr(SPRN_MMCR0, mmcr0);
> >  		}
> >  
> > +		/* Reload MMCRA to restore BHRB disable bit for POWER10 */
> > +		if (cpu_has_feature(CPU_FTR_ARCH_31))
> > +			mtspr(SPRN_MMCRA, mmcra_bhrb);
> > +
> >  		/*
> >  		 * DD2.2 and earlier need to set then clear bit 60 in MMCRA
> >  		 * to ensure the PMU starts running.
> 

--
Thanks and Regards
gautham.