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>

diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
index 29d4b6244a6f630515a01ba13cd9c64b26187f07..fbc37b2733e20aaf6a19fb3cb54b390ccf271bd6 100644 (file)
--- a/arch/arm64/kernel/pi/idreg-override.c
+++ b/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         },