Re: [PATCH v3 21/22] arm64: Delay enabling hardware DBM feature

[Suzuki K Poulose]

On 09/02/18 18:58, Dave Martin wrote:
> On Fri, Feb 09, 2018 at 05:55:12PM +0000, Suzuki K Poulose wrote:
> > We enable hardware DBM bit in a capable CPU, very early in the
> > boot via __cpu_setup. This doesn't give us a flexibility of
> > optionally disable the feature, as the clearing the bit
> > is a bit costly as the TLB can cache the settings. Instead,
> > we delay enabling the feature until the CPU is brought up
> > into the kernel. We use the feature capability mechanism
> > to handle it.
> >
> > The hardware DBM is a non-conflicting feature. i.e, the kernel
> > can safely run with a mix of CPUs with some using the feature
> > and the others don't. So, it is safe for a late CPU to have
> > this capability and enable it, even if the active CPUs don't.
> >
> > To get this handled properly by the infrastructure, we
> > unconditionally set the capability and only enable it
> > on CPUs which really have the feature. Also, we print the
> > feature detection from the "matches" call back to make sure
> > we don't mislead the user when none of the CPUs could use the
> > feature.
> >
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Dave Martin <dave.martin@xxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> > Changes since V2
> >   - Print the feature detection message only when at least one CPU
> >     is actually using it.


> > +static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,
> > +		       int __unused)
> > +{
> > +	static bool detected = false;
> > +	/*
> > +	 * DBM is a non-conflicting feature. i.e, the kernel can safely
> > +	 * run a mix of CPUs with and without the feature. So, we
> > +	 * unconditionally enable the capability to allow any late CPU
> > +	 * to use the feature. We only enable the control bits on the
> > +	 * CPU, if it actually supports.
> > +	 *
> > +	 * We have to make sure we print the "feature" detection only
> > +	 * when at least one CPU actually uses it. So check if this CPU
> > +	 * can actually use it and print the message exactly once.
> > +	 *
> > +	 * This is safe as all CPUs (including secondary CPUs - due to the
> > +	 * LOCAL_CPU scope - and the hotplugged CPUs - via verification)
> > +	 * goes through the "matches" check exactly once. Also if a CPU
> > +	 * matches the criteria, it is guaranteed that the CPU will turn
> > +	 * the DBM on, as the capability is unconditionally enabled.
> > +	 */
> > +	if (!detected && cpu_can_use_dbm(cap)) {
> > +		detected = true;
> > +		pr_info("detected feature: Hardware dirty bit management\n");
> > +	}
>
> Can we just do
>
> 	if (cpu_can_use_dbm(cap))
> 		pr_info_once(...);
>
> Then we can get rid of "detected".

The reason for open coding is the cost of cpu_can_use_dbm() with
addition of black listed CPUs in the next patch in the series.

Cheers
Suzuki