[PATCH 2/2] arm64/head: Disable MMU at EL2 before clearing HCR_EL2.E2H
Marc Zyngier
maz at kernel.org
Mon Apr 15 01:22:39 PDT 2024
On Mon, 15 Apr 2024 08:54:15 +0100,
Ard Biesheuvel <ardb+git at google.com> wrote:
>
> From: Ard Biesheuvel <ardb at kernel.org>
>
> 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.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
> arch/arm64/kernel/head.S | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index b8bbd72cb194..cb68adcabe07 100644
> --- 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
>
Acked-by: Marc Zyngier <maz at kernel.org>
M.
--
Without deviation from the norm, progress is not possible.
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