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BACKPORT: KVM: arm64: Remove VHE host restore of CPACR_EL1.SMEN

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[ 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:

  861262ab86 ("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:

  375110ab51 ("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 861262ab86.

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.

Bug: 411040189
Change-Id: Ia2fbb22a21da8e63f0a3b9a76d47ee2c987e2fa5
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-5-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
[Update for rework of flags storage -- broonie]
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
This commit is contained in:
Mark Rutland
2025-02-10 19:52:22 +00:00
committed by Will Deacon
parent c952e23cf8
commit c00c44bea2
2 changed files with 0 additions and 33 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
arch/arm64/include/asm/kvm_host.h
View File

@@ -693,8 +693,6 @@ struct kvm_vcpu_arch {
/* pKVM host vcpu state is dirty, needs resync */ /* pKVM host vcpu state is dirty, needs resync */
#define PKVM_HOST_STATE_DIRTY __vcpu_single_flag(iflags, BIT(7)) #define PKVM_HOST_STATE_DIRTY __vcpu_single_flag(iflags, BIT(7))
/* SME enabled for EL0 */
#define HOST_SME_ENABLED __vcpu_single_flag(sflags, BIT(1))
/* Physical CPU not in supported_cpus */ /* Physical CPU not in supported_cpus */
#define ON_UNSUPPORTED_CPU __vcpu_single_flag(sflags, BIT(2)) #define ON_UNSUPPORTED_CPU __vcpu_single_flag(sflags, BIT(2))
/* WFIT instruction trapped */ /* WFIT instruction trapped */

31
arch/arm64/kvm/fpsimd.c
View File

@@ -64,21 +64,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
*/ */
fpsimd_save_and_flush_cpu_state(); fpsimd_save_and_flush_cpu_state();
vcpu->arch.fp_state = FP_STATE_FREE; 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);
}
} }
/* /*
@@ -132,22 +117,6 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
local_irq_save(flags); 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->arch.fp_state == FP_STATE_GUEST_OWNED) {
if (vcpu_has_sve(vcpu)) { if (vcpu_has_sve(vcpu)) {
__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR); __vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);