Re: [PATCH v4 10/21] arm64: cpufeature: Use IDreg override in __read_sysreg_by_encoding()

[Suzuki K Poulose]

On 1/22/21 6:53 PM, Catalin Marinas wrote:
> On Mon, Jan 18, 2021 at 09:45:22AM +0000, Marc Zyngier wrote:
> > __read_sysreg_by_encoding() is used by a bunch of cpufeature helpers,
> > which should take the feature override into account. Let's do that.
> >
> > For a good measure (and because we are likely to need to further
> > down the line), make this helper available to the rest of the
> > non-modular kernel.
> >
> > Code that needs to know the *real* features of a CPU can still
> > use read_sysreg_s(), and find the bare, ugly truth.
> >
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>

> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index aaa075c6f029..48a011935d8c 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1149,14 +1149,17 @@ u64 read_sanitised_ftr_reg(u32 id)
> >   EXPORT_SYMBOL_GPL(read_sanitised_ftr_reg);
> >   
> >   #define read_sysreg_case(r)	\
> > -	case r:		return read_sysreg_s(r)
> > +	case r:		val = read_sysreg_s(r); break;
> >   
> >   /*
> >    * __read_sysreg_by_encoding() - Used by a STARTING cpu before cpuinfo is populated.
> >    * Read the system register on the current CPU
> >    */
> > -static u64 __read_sysreg_by_encoding(u32 sys_id)
> > +u64 __read_sysreg_by_encoding(u32 sys_id)
> >   {
> > +	struct arm64_ftr_reg *regp;
> > +	u64 val;
> > +
> >   	switch (sys_id) {
> >   	read_sysreg_case(SYS_ID_PFR0_EL1);
> >   	read_sysreg_case(SYS_ID_PFR1_EL1);
> > @@ -1199,6 +1202,14 @@ static u64 __read_sysreg_by_encoding(u32 sys_id)
> >   		BUG();
> >   		return 0;
> >   	}
> > +
> > +	regp  = get_arm64_ftr_reg(sys_id);
> > +	if (regp && regp->override_mask && regp->override_val) {
> > +		val &= ~*regp->override_mask;
> > +		val |= (*regp->override_val & *regp->override_mask);
> > +	}
> > +
> > +	return val;
>
> Ah, now the previous patch makes more sense. I don't particularly like
> this but I can't tell how to work around it. I was hoping that the
> overriding feature behaves more like a secondary CPU that limits all the
> overridden features. However, this approach would fail for FTR_EXACT
> cases (like PAC, though I wonder whether it fails already with your
> previous patch since the boot CPU value won't match the override, hence
> dropping to the safe one).

Correct !For FTR_EXACT, we dont want to override a value that is not safe, e.g PAC.
This is handled correctly in the previous patch and thus we are covered.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>