arm64: Add command-line override for ID_AA64MMFR0_EL1.ECV

It appears that relatively popular hardware out there implements
the CNTPOFF_EL2 variant of FEAT_ECV, advertises it via ID_AA64MMFR0_EL1,
but cannot be bothered to set SCR_EL3.ECVEn to 1.

You would probably think that "this is fine, EL3 will take the
trap on access to CNTPOFF_EL2 and flip the ECVEn bit", as that's
what a semi-decent firmware implementation would do.

But no. None of that. This particular implementation takes the trap,
considers its purpose in life, decides that it has none, and *RESETS*
the system.

Yes, x1e001de, I'm talking about you.

In order to allow this machine to be promoted slightly above the
level of a glorified door-stop, add a new "id_aa64mmfr0.ecv" override.
allowing the kernel to pretend this option was never there.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241021181434.1052974-1-maz@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Marc Zyngier

arch/arm64/kernel/pi/idreg-override.c

@@ -38,6 +38,15 @@ struct ftr_set_desc {
 
 #define FIELD(n, s, f)	{ .name = n, .shift = s, .width = 4, .filter = f }
 
+static const struct ftr_set_desc mmfr0 __prel64_initconst = {
+	.name		= "id_aa64mmfr0",
+	.override	= &id_aa64mmfr0_override,
+	.fields		= {
+		FIELD("ecv", ID_AA64MMFR0_EL1_ECV_SHIFT, NULL),
+		{}
+	},
+};
+
 static bool __init mmfr1_vh_filter(u64 val)
 {
 	/*
@@ -196,6 +205,7 @@ static const struct ftr_set_desc sw_features __prel64_initconst = {
 
 static const
 PREL64(const struct ftr_set_desc, reg) regs[] __prel64_initconst = {
+	{ &mmfr0	},
 	{ &mmfr1	},
 	{ &mmfr2	},
 	{ &pfr0 	},