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Message-Id: <20250325-stable-sve-6-1-v1-8-83259d427d84@kernel.org>
Date: Tue, 25 Mar 2025 18:48:22 +0000
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Oliver Upton <oliver.upton@...ux.dev>, Oleg Nesterov <oleg@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
kvmarm@...ts.linux.dev, kvmarm@...ts.cs.columbia.edu,
Mark Brown <broonie@...nel.org>, Mark Rutland <mark.rutland@....com>,
Fuad Tabba <tabba@...gle.com>
Subject: [PATCH 6.1 08/12] KVM: arm64: Remove VHE host restore of
CPACR_EL1.SMEN
From: Mark Rutland <mark.rutland@....com>
[ Upstream commit 407a99c4654e8ea65393f412c421a55cac539f5b ]
When KVM is in VHE mode, the host kernel tries to save and restore the
configuration of CPACR_EL1.SMEN (i.e. CPTR_EL2.SMEN when HCR_EL2.E2H=1)
across kvm_arch_vcpu_load_fp() and kvm_arch_vcpu_put_fp(), since the
configuration may be clobbered by hyp when running a vCPU. This logic
has historically been broken, and is currently redundant.
This logic was originally introduced in commit:
861262ab86270206 ("KVM: arm64: Handle SME host state when running guests")
At the time, the VHE hyp code would reset CPTR_EL2.SMEN to 0b00 when
returning to the host, trapping host access to SME state. Unfortunately,
this was unsafe as the host could take a softirq before calling
kvm_arch_vcpu_put_fp(), and if a softirq handler were to use kernel mode
NEON the resulting attempt to save the live FPSIMD/SVE/SME state would
result in a fatal trap.
That issue was limited to VHE mode. For nVHE/hVHE modes, KVM always
saved/restored the host kernel's CPACR_EL1 value, and configured
CPTR_EL2.TSM to 0b0, ensuring that host usage of SME would not be
trapped.
The issue above was incidentally fixed by commit:
375110ab51dec5dc ("KVM: arm64: Fix resetting SME trap values on reset for (h)VHE")
That commit changed the VHE hyp code to configure CPTR_EL2.SMEN to 0b01
when returning to the host, permitting host kernel usage of SME,
avoiding the issue described above. At the time, this was not identified
as a fix for commit 861262ab86270206.
Now that the host eagerly saves and unbinds its own FPSIMD/SVE/SME
state, there's no need to save/restore the state of the EL0 SME trap.
The kernel can safely save/restore state without trapping, as described
above, and will restore userspace state (including trap controls) before
returning to userspace.
Remove the redundant logic.
Signed-off-by: Mark Rutland <mark.rutland@....com>
Reviewed-by: Mark Brown <broonie@...nel.org>
Tested-by: Mark Brown <broonie@...nel.org>
Acked-by: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Fuad Tabba <tabba@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>
Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-5-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@...nel.org>
[Update for rework of flags storage -- broonie]
Signed-off-by: Mark Brown <broonie@...nel.org>
---
arch/arm64/include/asm/kvm_host.h | 2 --
arch/arm64/kvm/fpsimd.c | 31 -------------------------------
2 files changed, 33 deletions(-)
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 181e49120e0c4027aa52dd389de13f9ce5cd7b57..757f4dea1e563657eb5c79e624e4b91f514a113a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -556,8 +556,6 @@ struct kvm_vcpu_arch {
/* Save TRBE context if active */
#define DEBUG_STATE_SAVE_TRBE __vcpu_single_flag(iflags, BIT(6))
-/* SME enabled for EL0 */
-#define HOST_SME_ENABLED __vcpu_single_flag(sflags, BIT(1))
/* Physical CPU not in supported_cpus */
#define ON_UNSUPPORTED_CPU __vcpu_single_flag(sflags, BIT(2))
/* WFIT instruction trapped */
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 8d073a37c266db3dc2726c6a0bb39c7e2586f53f..df050e4d3562d2ef36b7c27602a6feaa431dfa93 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -87,21 +87,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
*/
fpsimd_save_and_flush_cpu_state();
vcpu->arch.fp_state = FP_STATE_FREE;
-
- /*
- * We don't currently support SME guests but if we leave
- * things in streaming mode then when the guest starts running
- * FPSIMD or SVE code it may generate SME traps so as a
- * special case if we are in streaming mode we force the host
- * state to be saved now and exit streaming mode so that we
- * don't have to handle any SME traps for valid guest
- * operations. Do this for ZA as well for now for simplicity.
- */
- if (system_supports_sme()) {
- vcpu_clear_flag(vcpu, HOST_SME_ENABLED);
- if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)
- vcpu_set_flag(vcpu, HOST_SME_ENABLED);
- }
}
/*
@@ -162,22 +147,6 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
local_irq_save(flags);
- /*
- * If we have VHE then the Hyp code will reset CPACR_EL1 to
- * CPACR_EL1_DEFAULT and we need to reenable SME.
- */
- if (has_vhe() && system_supports_sme()) {
- /* Also restore EL0 state seen on entry */
- if (vcpu_get_flag(vcpu, HOST_SME_ENABLED))
- sysreg_clear_set(CPACR_EL1, 0,
- CPACR_EL1_SMEN_EL0EN |
- CPACR_EL1_SMEN_EL1EN);
- else
- sysreg_clear_set(CPACR_EL1,
- CPACR_EL1_SMEN_EL0EN,
- CPACR_EL1_SMEN_EL1EN);
- }
-
if (vcpu->arch.fp_state == FP_STATE_GUEST_OWNED) {
if (vcpu_has_sve(vcpu)) {
__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);
--
2.39.5
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