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

Suzuki K Poulose
2021-01-23T16:06:09Z

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@kernel.org>

>> 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@arm.com>