arm64/head: Disable MMU at EL2 before clearing HCR_EL2.E2H

Even though the boot protocol stipulates otherwise, an exception has
been made for the EFI stub, and entering the core kernel with the MMU
enabled is permitted. This allows a substantial amount of cache
maintenance to be elided, wich is significant when fast boot times are
critical (e.g., for booting micro-VMs)

Once the initial ID map has been populated, the MMU is disabled as part
of the logic sequence that puts all system registers into a known state.
Any code that needs to execute within the window where the MMU is off is
cleaned to the PoC explicitly, which includes all of HYP text when
entering at EL2.

However, the current sequence of initializing the EL2 system registers
is not safe: HCR_EL2 is set to its nVHE initial state before SCTLR_EL2
is reprogrammed, and this means that a VHE-to-nVHE switch may occur
while the MMU is enabled. This switch causes some system registers as
well as page table descriptors to be interpreted in a different way,
potentially resulting in spurious exceptions relating to MMU
translation.

So disable the MMU explicitly first when entering in EL2 with the MMU
and caches enabled.

Fixes: 617861703830 ("efi: arm64: enter with MMU and caches enabled")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: <stable@vger.kernel.org> # 6.3.x
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240415075412.2347624-6-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index b8bbd72cb1944f71e6b774032a2fa0bd08c712ab..cb68adcabe0789201aa4b37befaa686c67dc7db7 100644 (file)
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -289,6 +289,11 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
        adr_l   x1, __hyp_text_end
        adr_l   x2, dcache_clean_poc
        blr     x2
+
+       mov_q   x0, INIT_SCTLR_EL2_MMU_OFF
+       pre_disable_mmu_workaround
+       msr     sctlr_el2, x0
+       isb
 0:
        mov_q   x0, HCR_HOST_NVHE_FLAGS