linux-kernel - Re: [PATCH v9 05/30] arm64/fpsimd: Determine maximum virtualisable SME vector length

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Message-ID: <CA+EHjTycP4Wz0V7S8hzWygpeXGzeehTL8RFfit7Eaq4rT+Eu+Q@mail.gmail.com>
Date: Wed, 7 Jan 2026 19:25:47 +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 05/30] arm64/fpsimd: Determine maximum virtualisable
 SME vector length

Hi Mark,

On Tue, 23 Dec 2025 at 01:21, Mark Brown <broonie@...nel.org> wrote:
>
> As with SVE we can only virtualise SME vector lengths that are supported by
> all CPUs in the system, implement similar checks to those for SVE. Since
> unlike SVE there are no specific vector lengths that are architecturally
> required the handling is subtly different, we report a system where this
> happens with a maximum vector length of -1.
>
> Signed-off-by: Mark Brown <broonie@...nel.org>
> ---
>  arch/arm64/kernel/fpsimd.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index f4e8cee00198..22f8397c67f0 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -1257,7 +1257,8 @@ void cpu_enable_sme(const struct arm64_cpu_capabilities *__always_unused p)
>  void __init sme_setup(void)
>  {
>         struct vl_info *info = &vl_info[ARM64_VEC_SME];
> -       int min_bit, max_bit;
> +       DECLARE_BITMAP(tmp_map, SVE_VQ_MAX);
> +       int min_bit, max_bit, b;
>
>         if (!system_supports_sme())
>                 return;
> @@ -1288,12 +1289,32 @@ void __init sme_setup(void)
>          */
>         set_sme_default_vl(find_supported_vector_length(ARM64_VEC_SME, 32));
>
> +       bitmap_andnot(tmp_map, info->vq_partial_map, info->vq_map,
> +                     SVE_VQ_MAX);
> +
> +       b = find_last_bit(tmp_map, SVE_VQ_MAX);
> +       if (b >= SVE_VQ_MAX)
> +               /* All VLs virtualisable */
> +               info->max_virtualisable_vl = SVE_VQ_MAX;
> +       else if (b == SVE_VQ_MAX - 1)
> +               /* No virtualisable VLs */
> +               info->max_virtualisable_vl = -1;

I'm not sure about -1 as the "No virtualisable VLs" value. Unless I've
missed something, this value gets used without being checked,
potentially even assigned to an unsigned int:

> kvm_max_vl[ARM64_VEC_SME] = sme_max_virtualisable_vl();

Cheers,
/fuad


> +       else
> +               info->max_virtualisable_vl = sve_vl_from_vq(__bit_to_vq(b +  1));
> +
> +       if (info->max_virtualisable_vl > info->max_vl)
> +               info->max_virtualisable_vl = info->max_vl;
> +
>         pr_info("SME: minimum available vector length %u bytes per vector\n",
>                 info->min_vl);
>         pr_info("SME: maximum available vector length %u bytes per vector\n",
>                 info->max_vl);
>         pr_info("SME: default vector length %u bytes per vector\n",
>                 get_sme_default_vl());
> +
> +       /* KVM decides whether to support mismatched systems. Just warn here: */
> +       if (info->max_virtualisable_vl < info->max_vl)
> +               pr_warn("SME: unvirtualisable vector lengths present\n");
>  }
>
>  void sme_suspend_exit(void)
>
> --
> 2.47.3
>