linux-kernel - [RFC 5/8] KVM: arm64: Explicitly handle MDSELR

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Date: Fri,  5 Apr 2024 13:30:05 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: linux-arm-kernel@...ts.infradead.org
Cc: Anshuman Khandual <anshuman.khandual@....com>,
	Jonathan Corbet <corbet@....net>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [RFC 5/8] KVM: arm64: Explicitly handle MDSELR_EL1 traps as UNDEFINED

Currently read_sanitised_id_aa64dfr0_el1() caps the ID_AA64DFR0.DebugVer to
ID_AA64DFR0_DebugVer_V8P8, resulting in FEAT_Debugv8p9 not being exposed to
the guest. MDSELR_EL1 register access in the guest, is currently trapped by
the existing configuration of the fine-grained traps.

As the register is not described in sys_reg_descs[] table emulate_sys_reg()
will warn that this is unknown access before injecting an UNDEFINED
exception into the guest. Any well-behaved guests shouldn't try to use this
register, but any badly-behaved guests could, thus resulting in unnecessary
warnings. To avoid such warnings, access to MDSELR_EL1 should be explicitly
handled as UNDEFINED via updating sys_reg_desc[] as required.

Cc: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>
Cc: James Morse <james.morse@....com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org
Cc: kvmarm@...ts.linux.dev
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
---
 arch/arm64/kvm/sys_regs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c9f4f387155f..2956bdcd358e 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2203,6 +2203,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	{ SYS_DESC(SYS_MDSCR_EL1), trap_debug_regs, reset_val, MDSCR_EL1, 0 },
 	DBG_BCR_BVR_WCR_WVR_EL1(2),
 	DBG_BCR_BVR_WCR_WVR_EL1(3),
+	{ SYS_DESC(SYS_MDSELR_EL1), undef_access },
 	DBG_BCR_BVR_WCR_WVR_EL1(4),
 	DBG_BCR_BVR_WCR_WVR_EL1(5),
 	DBG_BCR_BVR_WCR_WVR_EL1(6),
-- 
2.25.1