[PATCH] KVM: arm64: pkvm: Fixup boot mode to reflect that the kernel resumes from EL1

From: Marc Zyngier <maz@kernel.org>
To: <kvmarm@lists.cs.columbia.edu>, <kvmarm@lists.linux.dev>,
	kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc: James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Quentin Perret <qperret@google.com>,
	Will Deacon <will@kernel.org>,
	Vincent Donnefort <vdonnefort@google.com>
Subject: [PATCH] KVM: arm64: pkvm: Fixup boot mode to reflect that the kernel resumes from EL1
Date: Tue, 11 Oct 2022 17:54:00 +0100	[thread overview]
Message-ID: <20221011165400.1241729-1-maz@kernel.org> (raw)

The kernel has an awfully complicated boot sequence in order to cope
with the various EL2 configurations, including those that "enhanced"
the architecture. We go from EL2 to EL1, then back to EL2, staying
at EL2 if VHE capable and otherwise go back to EL1.

Here's a paracetamol tablet for you.

The cpu_resume path follows the same logic, because coming up with
two versions of a square wheel is hard.

However, things aren't this straightforward with pKVM, as the host
resume path is always proxied by the hypervisor, which means that
the kernel is always entered at EL1. Which contradicts what the
__boot_cpu_mode[] array contains (it obviously says EL2).

This thus triggers a HVC call from EL1 to EL2 in a vain attempt
to upgrade from EL1 to EL2 VHE, which we are, funnily enough,
reluctant to grant to the host kernel. This is also completely
unexpected, and puzzles your average EL2 hacker.

Address it by fixing up the boot mode at the point the host gets
deprivileged. is_hyp_mode_available() and co already have a static
branch to deal with this, making it pretty safe.

Reported-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/arm.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index b6c9bfa8492f..cf075c9b9ab1 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -2107,6 +2107,17 @@ static int pkvm_drop_host_privileges(void)
 	 * once the host stage 2 is installed.
 	 */
 	static_branch_enable(&kvm_protected_mode_initialized);
+
+	/*
+	 * Fixup the boot mode so that we don't take spurious round
+	 * trips via EL2 on cpu_resume. Flush to the PoC for a good
+	 * measure, so that it can be observed by a CPU coming out of
+	 * suspend with the MMU off.
+	 */
+	__boot_cpu_mode[0] = __boot_cpu_mode[1] = BOOT_CPU_MODE_EL1;
+	dcache_clean_poc((unsigned long)__boot_cpu_mode,
+			 (unsigned long)(__boot_cpu_mode + 2));
+
 	on_each_cpu(_kvm_host_prot_finalize, &ret, 1);
 	return ret;
 }
-- 
2.34.1


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