[PATCH v2 4/8] KVM: arm64: Remove VHE host restore of CPACR_EL1.SMEN

[Will Deacon]

On Thu, Feb 06, 2025 at 02:10:58PM +0000, Mark Rutland wrote:
> 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 at arm.com>
> Reviewed-by: Mark Brown <broonie at kernel.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Fuad Tabba <tabba at google.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Mark Brown <broonie at kernel.org>
> Cc: Oliver Upton <oliver.upton at linux.dev>
> Cc: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/include/asm/kvm_host.h |  1 -
>  arch/arm64/kvm/fpsimd.c           | 21 ---------------------
>  2 files changed, 22 deletions(-)

Acked-by: Will Deacon <will at kernel.org>

Will