Re: [PATCH v3 14/22] arm64: capabilities: Add support for features enabled early

[Suzuki K Poulose]

On 12/02/18 17:17, Dave Martin wrote:
> On Fri, Feb 09, 2018 at 05:54:57PM +0000, Suzuki K Poulose wrote:
> > The kernel detects and uses some of the features based on the boot
> > CPU and expects that all the following CPUs conform to it. e.g,
> > with VHE and the boot CPU running at EL2, the kernel decides to
> > keep the kernel running at EL2. If another CPU is brought up without
> > this capability, we use custom hooks (via check_early_cpu_features())
> > to handle it. To handle such capabilities add support for detecting
> > and enabling capabilities based on the boot CPU.

> > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> > index 383c69c95f23..5f56a8342065 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
> > @@ -104,7 +104,7 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
> >    *    some checks at runtime. This could be, e.g, checking the value of a field
> >    *    in CPU ID feature register or checking the cpu model. The capability
> >    *    provides a call back ( @matches() ) to perform the check.
> > - *    Scope defines how the checks should be performed. There are two cases:
> > + *    Scope defines how the checks should be performed. There are three cases:
> >    *
> >    *     a) SCOPE_LOCAL_CPU: check all the CPUs and "detect" if at least one
> >    *        matches. This implies, we have to run the check on all the booting
> > @@ -117,6 +117,11 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
> >    *        field in one of the CPU ID feature registers, we use the sanitised
> >    *        value of the register from the CPU feature infrastructure to make
> >    *        the decision.
> > + *		Or
> > + *     c) SCOPE_BOOT_CPU: Check only on the primary boot CPU to detect the feature.
> > + *        This category is for features that are "finalised" (or used) by the kernel
> > + *        very early even before the SMP cpus are brought up.
> > + *
>
> Nit: the overlong lines bring no benefit here.  Please wrap them if
> possible -- but to avoid patch churn only bother for lines actually
> changed/added by this patch.

Sure

> >   static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
> > @@ -1277,12 +1277,21 @@ __enable_cpu_capabilities(const struct arm64_cpu_capabilities *caps, u16 scope_m
> >   
> >   		if (caps->cpu_enable) {
> >   			/*
> > -			 * Use stop_machine() as it schedules the work allowing
> > -			 * us to modify PSTATE, instead of on_each_cpu() which
> > -			 * uses an IPI, giving us a PSTATE that disappears when
> > -			 * we return.
> > +			 * If we are dealing with a boot CPU capability, we
> > +			 * have to enable this only on the Boot CPU, where it
> > +			 * is detected. All the secondaries enable it via
> > +			 * check_local_cpu_capabilities().
>
> I found this confusing to read, because it's not 100% clear whether the
> "If we are dealing with a boot CPU capability" applies to the second
> sentence as well.
>
> Maybe this would be clearer as:
>
> "Capabilities with SCOPE_BOOT_CPU are finalised before any secondary
> CPU boots.  Thus, each secondary will enable the capability as
> appropriate via check_local_cpu_capabilities().  The only exception is
> the boot CPU, for which the capability must be enabled here.  This
> approach avoids costly stop_machine() calls for this case."
>
> Thoughts?

Definitely better, will change it.


> > @@ -1362,6 +1371,12 @@ static void check_early_cpu_features(void)
> >   {
> >   	verify_cpu_run_el();
> >   	verify_cpu_asid_bits();
> > +	/*
> > +	 * Early features are used by the kernel already. If there
> > +	 * is a conflict, we cannot proceed further.
> > +	 */
> > +	if (!verify_local_cpu_caps(SCOPE_BOOT_CPU))
> > +		cpu_panic_kernel();
> >   }
> >   
> >   static void
> > @@ -1403,9 +1418,8 @@ static void verify_sve_features(void)
> >    */
> >   static void verify_local_cpu_capabilities(void)
> >   {
>
> Nit: Maybe add a comment saying where SCOPE_BOOT_CPU capabilities are
> checked.

Ok

> > -	if (!verify_local_cpu_caps(SCOPE_ALL))
> > +	if (!verify_local_cpu_caps(SCOPE_ALL & ~SCOPE_BOOT_CPU))
> >   		cpu_die_early();
> > -
>
> Nit: keep blank line?
>
> Otherwise it looks like the if() falls through, where really
> cpu_die_early() does not return.

ok

> >   void __init setup_cpu_features(void)
> > --
> > 2.14.3
>
> With fair consideration given to the nits above:
>
> Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx>

Cheers
Suzuki