linux-kernel - Re: [PATCH v9 03/30] arm64/fpsimd: Decide to save ZT0 and streaming mode FFR at bind time

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Message-ID: <CA+EHjTyZg=WH4GmAXzmNroAPVtRUM=LMvNWc38S4_Oh7UQi78w@mail.gmail.com>
Date: Wed, 7 Jan 2026 19:25:09 +0000
From: Fuad Tabba <tabba@...gle.com>
To: Mark Brown <broonie@...nel.org>
Cc: Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>, 
	Catalin Marinas <catalin.marinas@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Will Deacon <will@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
	Shuah Khan <shuah@...nel.org>, Oliver Upton <oupton@...nel.org>, Dave Martin <Dave.Martin@....com>, 
	Mark Rutland <mark.rutland@....com>, Ben Horgan <ben.horgan@....com>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Peter Maydell <peter.maydell@...aro.org>, 
	Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH v9 03/30] arm64/fpsimd: Decide to save ZT0 and streaming
 mode FFR at bind time

On Tue, 23 Dec 2025 at 01:21, Mark Brown <broonie@...nel.org> wrote:
>
> Some parts of the SME state are optional, enabled by additional features
> on top of the base FEAT_SME and controlled with enable bits in SMCR_ELx. We
> unconditionally enable these for the host but for KVM we will allow the
> feature set exposed to guests to be restricted by the VMM. These are the
> FFR register (FEAT_SME_FA64) and ZT0 (FEAT_SME2).
>
> We defer saving of guest floating point state for non-protected guests to
> the host kernel. We also want to avoid having to reconfigure the guest
> floating point state if nothing used the floating point state while running
> the host. If the guest was running with the optional features disabled then
> traps will be enabled for them so the host kernel will need to skip
> accessing that state when saving state for the guest.
>
> Support this by moving the decision about saving this state to the point
> where we bind floating point state to the CPU, adding a new variable to
> the cpu_fp_state which uses the enable bits in SMCR_ELx to flag which
> features are enabled.
>
> Signed-off-by: Mark Brown <broonie@...nel.org>

Reviewed-by: Fuad Tabba <tabba@...gle.com>

Cheers,
/fuad


> ---
>  arch/arm64/include/asm/fpsimd.h |  1 +
>  arch/arm64/kernel/fpsimd.c      | 10 ++++++++--
>  arch/arm64/kvm/fpsimd.c         |  1 +
>  3 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> index ece65061dea0..146c1af55e22 100644
> --- a/arch/arm64/include/asm/fpsimd.h
> +++ b/arch/arm64/include/asm/fpsimd.h
> @@ -87,6 +87,7 @@ struct cpu_fp_state {
>         void *sme_state;
>         u64 *svcr;
>         u64 *fpmr;
> +       u64 sme_features;
>         unsigned int sve_vl;
>         unsigned int sme_vl;
>         enum fp_type *fp_type;
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index be4499ff6498..887fce177c92 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -490,12 +490,12 @@ static void fpsimd_save_user_state(void)
>
>                 if (*svcr & SVCR_ZA_MASK)
>                         sme_save_state(last->sme_state,
> -                                      system_supports_sme2());
> +                                      last->sme_features & SMCR_ELx_EZT0);
>
>                 /* If we are in streaming mode override regular SVE. */
>                 if (*svcr & SVCR_SM_MASK) {
>                         save_sve_regs = true;
> -                       save_ffr = system_supports_fa64();
> +                       save_ffr = last->sme_features & SMCR_ELx_FA64;
>                         vl = last->sme_vl;
>                 }
>         }
> @@ -1671,6 +1671,12 @@ static void fpsimd_bind_task_to_cpu(void)
>         last->to_save = FP_STATE_CURRENT;
>         current->thread.fpsimd_cpu = smp_processor_id();
>
> +       last->sme_features = 0;
> +       if (system_supports_fa64())
> +               last->sme_features |= SMCR_ELx_FA64;
> +       if (system_supports_sme2())
> +               last->sme_features |= SMCR_ELx_EZT0;
> +
>         /*
>          * Toggle SVE and SME trapping for userspace if needed, these
>          * are serialsied by ret_to_user().
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index 15e17aca1dec..9158353d8be3 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -80,6 +80,7 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
>                 fp_state.svcr = __ctxt_sys_reg(&vcpu->arch.ctxt, SVCR);
>                 fp_state.fpmr = __ctxt_sys_reg(&vcpu->arch.ctxt, FPMR);
>                 fp_state.fp_type = &vcpu->arch.fp_type;
> +               fp_state.sme_features = 0;
>
>                 if (vcpu_has_sve(vcpu))
>                         fp_state.to_save = FP_STATE_SVE;
>
> --
> 2.47.3
>