[PATCH 5/5] arm64: Revamp HCR_EL2.E2H RES1 detection
From: Colton Lewis <hidden>
Date: 2026-07-01 20:43:49
Also in:
kvmarm, lkml, stable
Subsystem:
arm64 port (aarch64 architecture), the rest · Maintainers:
Catalin Marinas, Will Deacon, Linus Torvalds
From: Marc Zyngier <maz@kernel.org> [ Upstream commit ca88ecdce5f5127ef2ee241b12b23a7e03c6210f ] We currently have two ways to identify CPUs that only implement FEAT_VHE and not FEAT_E2H0: - either they advertise it via ID_AA64MMFR4_EL1.E2H0, - or the HCR_EL2.E2H bit is RAO/WI However, there is a third category of "cpus" that fall between these two cases: on CPUs that do not implement FEAT_FGT, it is IMPDEF whether an access to ID_AA64MMFR4_EL1 can trap to EL2 when the register value is zero. A consequence of this is that on systems such as Neoverse V2, a NV guest cannot reliably detect that it is in a VHE-only configuration (E2H is writable, and ID_AA64MMFR0_EL1 is 0), despite the hypervisor's best effort to repaint the id register. Replace the RAO/WI test by a sequence that makes use of the VHE register remnapping between EL1 and EL2 to detect this situation, and work out whether we get the VHE behaviour even after having set HCR_EL2.E2H to 0. This solves the NV problem, and provides a more reliable acid test for CPUs that do not completely follow the letter of the architecture while providing a RES1 behaviour for HCR_EL2.E2H. Suggested-by: Mark Rutland <mark.rutland@arm.com> Acked-by: Mark Rutland <mark.rutland@arm.com> Acked-by: Catalin Marinas <catalin.marinas@arm.com> Reviewed-by: Oliver Upton <redacted> Tested-by: Jan Kotas <redacted> Signed-off-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/15A85F2B-1A0C-4FA7-9FE4-EEC2203CC09E@global.cadence.com (local) [ Backport: Resolved conflict in arch/arm64/include/asm/el2_setup.h by replacing msr_hcr_el2 macro usages with raw msr hcr_el2 (since the macro is missing in 6.6.y). ] --- arch/arm64/include/asm/el2_setup.h | 38 +++++++++++++++++++++++++----- 1 file changed, 32 insertions(+), 6 deletions(-)
diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
index 3498dc5d02c18..38d32116a23eb 100644
--- a/arch/arm64/include/asm/el2_setup.h
+++ b/arch/arm64/include/asm/el2_setup.h@@ -24,22 +24,48 @@ * ID_AA64MMFR4_EL1.E2H0 < 0. On such CPUs HCR_EL2.E2H is RES1, but it * can reset into an UNKNOWN state and might not read as 1 until it has * been initialized explicitly. - * - * Fruity CPUs seem to have HCR_EL2.E2H set to RAO/WI, but - * don't advertise it (they predate this relaxation). - * * Initalize HCR_EL2.E2H so that later code can rely upon HCR_EL2.E2H * indicating whether the CPU is running in E2H mode. */ mrs_s x1, SYS_ID_AA64MMFR4_EL1 sbfx x1, x1, #ID_AA64MMFR4_EL1_E2H0_SHIFT, #ID_AA64MMFR4_EL1_E2H0_WIDTH cmp x1, #0 - b.ge .LnVHE_\@ + b.lt .LnE2H0_\@ + /* + * Unfortunately, HCR_EL2.E2H can be RES1 even if not advertised + * as such via ID_AA64MMFR4_EL1.E2H0: + * + * - Fruity CPUs predate the !FEAT_E2H0 relaxation, and seem to + * have HCR_EL2.E2H implemented as RAO/WI. + * + * - On CPUs that lack FEAT_FGT, a hypervisor can't trap guest + * reads of ID_AA64MMFR4_EL1 to advertise !FEAT_E2H0. NV + * guests on these hosts can write to HCR_EL2.E2H without + * trapping to the hypervisor, but these writes have no + * functional effect. + * + * Handle both cases by checking for an essential VHE property + * (system register remapping) to decide whether we're + * effectively VHE-only or not. + */ + msr hcr_el2, x0 // Setup HCR_EL2 as nVHE + isb + mov x1, #1 // Write something to FAR_EL1 + msr far_el1, x1 + isb + mov x1, #2 // Try to overwrite it via FAR_EL2 + msr far_el2, x1 + isb + mrs x1, far_el1 // If we see the latest write in FAR_EL1, + cmp x1, #2 // we can safely assume we are VHE only. + b.ne .LnVHE_\@ // Otherwise, we know that nVHE works. + +.LnE2H0_\@: orr x0, x0, #HCR_E2H -.LnVHE_\@: msr hcr_el2, x0 isb +.LnVHE_\@: .endm .macro __init_el2_sctlr
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2.55.0.rc2.803.g1fd1e6609c-goog