linux-kernel - [PATCH v1 1/2] KVM: arm64: Allow BT field in ID_AA64PFR1

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Date: Mon, 17 Jun 2024 03:51:30 -0400
From: Shaoqin Huang <shahuang@...hat.com>
To: Oliver Upton <oliver.upton@...ux.dev>,
	Marc Zyngier <maz@...nel.org>,
	kvmarm@...ts.linux.dev
Cc: Shaoqin Huang <shahuang@...hat.com>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v1 1/2] KVM: arm64: Allow BT field in ID_AA64PFR1_EL1 writable

When migrating between some of the ARM machines, the BT field value in
ID_AA64PFR1_EL1 register is different and not writable. This causes
migration failed.

The BT field means Branch Target Identification mechanism support in
AArch64 state. The value 0 means BT is not implemented, the value 1
means BT is implemented.

Here is an example of the migration if the BT value is different:
* On Migration Src, the BT value is 0.
* On Migration Dst, the BT value is 1.

As it defined in the ftr_id_aa64dfr0, the samller value is safe. So if
we make the BT field writable, on the Migration Dst the BT field will
be overrided with value 0.
---
 arch/arm64/kvm/sys_regs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c9f4f387155f..958e4d251535 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2292,7 +2292,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 		   ID_AA64PFR0_EL1_GIC |
 		   ID_AA64PFR0_EL1_AdvSIMD |
 		   ID_AA64PFR0_EL1_FP), },
-	ID_SANITISED(ID_AA64PFR1_EL1),
+	ID_WRITABLE(ID_AA64PFR1_EL1, ID_AA64PFR1_EL1_BT),
 	ID_UNALLOCATED(4,2),
 	ID_UNALLOCATED(4,3),
 	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
-- 
2.40.1