linux-kernel - [PATCH v2 2/2] KVM: selftests: arm64: Explicitly set the page attrs to Inner-Shareable

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Message-ID: <20250405001042.1470552-3-rananta@google.com>
Date: Sat,  5 Apr 2025 00:10:42 +0000
From: Raghavendra Rao Ananta <rananta@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>
Cc: Raghavendra Rao Anata <rananta@...gle.com>, Mingwei Zhang <mizhang@...gle.com>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Oliver Upton <oupton@...gle.com>
Subject: [PATCH v2 2/2] KVM: selftests: arm64: Explicitly set the page attrs
 to Inner-Shareable

Atomic instructions such as 'ldset' over (global) variables in the guest
is observed to cause an EL1 data abort with FSC 0x35 (IMPLEMENTATION
DEFINED fault (Unsupported Exclusive or Atomic access)). The observation
was particularly apparent on Neoverse-N3.

According to ARM ARM DDI0487L.a B2.2.6 (Possible implementation
restrictions on using atomic instructions), atomic instructions are
architecturally guaranteed for Inner Shareable and Outer Shareable
attributes. For Non-Shareable attribute, the atomic instructions are
not atomic and issuing such an instruction can lead to the FSC
mentioned in this case (among other things).

Moreover, according to DDI0487L.a C3.2.6 (Single-copy atomic 64-byte
load/store), it is implementation defined that a data abort with the
mentioned FSC is reported for the first stage of translation that
provides an inappropriate memory type. It's likely that Neoverse-N3
chose to implement these two and why we see an FSC of 0x35 in EL1 upon
executing atomic instructions.

ARM64 KVM selftests sets no shareable attributes, which makes them
Non-Shareable by default. Hence, explicitly set them as Inner-Shareable
to fix this issue.

Suggested-by: Oliver Upton <oupton@...gle.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
---
 tools/testing/selftests/kvm/include/arm64/processor.h | 1 +
 tools/testing/selftests/kvm/lib/arm64/processor.c     | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/tools/testing/selftests/kvm/include/arm64/processor.h b/tools/testing/selftests/kvm/include/arm64/processor.h
index 7d88ff22013a..b0fc0f945766 100644
--- a/tools/testing/selftests/kvm/include/arm64/processor.h
+++ b/tools/testing/selftests/kvm/include/arm64/processor.h
@@ -113,6 +113,7 @@
 #define PMD_TYPE_TABLE		BIT(1)
 #define PTE_TYPE_PAGE		BIT(1)

+#define PTE_SHARED		(UL(3) << 8) /* SH[1:0], inner shareable */
 #define PTE_AF			BIT(10)

 #define PTE_ADDR_MASK(page_shift)	GENMASK(47, (page_shift))
diff --git a/tools/testing/selftests/kvm/lib/arm64/processor.c b/tools/testing/selftests/kvm/lib/arm64/processor.c
index da5802c8a59c..9d69904cb608 100644
--- a/tools/testing/selftests/kvm/lib/arm64/processor.c
+++ b/tools/testing/selftests/kvm/lib/arm64/processor.c
@@ -172,6 +172,9 @@ static void _virt_pg_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr,
 	}

 	pg_attr = PTE_AF | PTE_ATTRINDX(attr_idx) | PTE_TYPE_PAGE | PTE_VALID;
+	if (!use_lpa2_pte_format(vm))
+		pg_attr |= PTE_SHARED;
+
 	*ptep = addr_pte(vm, paddr, pg_attr);
 }

-- 
2.49.0.504.g3bcea36a83-goog