Re: [RFC PATCH v2 06/23] arm64/sve: Check SVE virtualisability

["Alex Bennée" <alex.bennee@linaro.org>]

Dave Martin <Dave.Martin@arm.com> writes:

> Due to the way the effective SVE vector length is controlled and
> trapped at different exception levels, certain mismatches in the
> sets of vector lengths supported by different physical CPUs in the
> system may prevent straightforward virtualisation of SVE at parity
> with the host.
>
> This patch analyses the extent to which SVE can be virtualised
> safely without interfering with migration of vcpus between physical
> CPUs, and rejects late secondary CPUs that would erode the
> situation further.
>
> It is left up to KVM to decide what to do with this information.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
>
> Changes since RFCv1:
>
>  * The analysis done by this patch is the same as in the previous
>    version, but the commit message the printks etc. have been reworded
>    to avoid the suggestion that KVM is expected to work on a system with
>    mismatched SVE implementations.
> ---
>  arch/arm64/include/asm/fpsimd.h |  1 +
>  arch/arm64/kernel/cpufeature.c  |  2 +-
>  arch/arm64/kernel/fpsimd.c      | 87 +++++++++++++++++++++++++++++++++++------
>  3 files changed, 76 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> index dd1ad39..964adc9 100644
> --- a/arch/arm64/include/asm/fpsimd.h
> +++ b/arch/arm64/include/asm/fpsimd.h
> @@ -87,6 +87,7 @@ extern void sve_kernel_enable(const struct arm64_cpu_capabilities *__unused);
>  extern u64 read_zcr_features(void);
>
>  extern int __ro_after_init sve_max_vl;
> +extern int __ro_after_init sve_max_virtualisable_vl;
>
>  #ifdef CONFIG_ARM64_SVE
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index e238b79..aa1a55b 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1531,7 +1531,7 @@ static void verify_sve_features(void)
>  	unsigned int len = zcr & ZCR_ELx_LEN_MASK;
>
>  	if (len < safe_len || sve_verify_vq_map()) {
> -		pr_crit("CPU%d: SVE: required vector length(s) missing\n",
> +		pr_crit("CPU%d: SVE: vector length support mismatch\n",
>  			smp_processor_id());
>  		cpu_die_early();
>  	}
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 42aa154..d28042b 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -18,6 +18,7 @@
>   */
>
>  #include <linux/bitmap.h>
> +#include <linux/bitops.h>
>  #include <linux/bottom_half.h>
>  #include <linux/bug.h>
>  #include <linux/cache.h>
> @@ -48,6 +49,7 @@
>  #include <asm/sigcontext.h>
>  #include <asm/sysreg.h>
>  #include <asm/traps.h>
> +#include <asm/virt.h>
>
>  #define FPEXC_IOF	(1 << 0)
>  #define FPEXC_DZF	(1 << 1)
> @@ -130,14 +132,18 @@ static int sve_default_vl = -1;
>
>  /* Maximum supported vector length across all CPUs (initially poisoned) */
>  int __ro_after_init sve_max_vl = SVE_VL_MIN;
> +int __ro_after_init sve_max_virtualisable_vl = SVE_VL_MIN;
>  /* Set of available vector lengths, as vq_to_bit(vq): */
>  static __ro_after_init DECLARE_BITMAP(sve_vq_map, SVE_VQ_MAX);
> +/* Set of vector lengths present on at least one cpu: */
> +static __ro_after_init DECLARE_BITMAP(sve_vq_partial_map, SVE_VQ_MAX);
>  static void __percpu *efi_sve_state;
>
>  #else /* ! CONFIG_ARM64_SVE */
>
>  /* Dummy declaration for code that will be optimised out: */
>  extern __ro_after_init DECLARE_BITMAP(sve_vq_map, SVE_VQ_MAX);
> +extern __ro_after_init DECLARE_BITMAP(sve_vq_partial_map, SVE_VQ_MAX);
>  extern void __percpu *efi_sve_state;
>
>  #endif /* ! CONFIG_ARM64_SVE */
> @@ -623,11 +629,8 @@ int sve_get_current_vl(void)
>  	return sve_prctl_status(0);
>  }
>
> -/*
> - * Bitmap for temporary storage of the per-CPU set of supported vector lengths
> - * during secondary boot.
> - */
> -static DECLARE_BITMAP(sve_secondary_vq_map, SVE_VQ_MAX);
> +/* Bitmaps for temporary storage during manipulation of vector length sets */
> +static DECLARE_BITMAP(sve_tmp_vq_map, SVE_VQ_MAX);

This seems odd as a local global, why not declared locally when used?

>
>  static void sve_probe_vqs(DECLARE_BITMAP(map, SVE_VQ_MAX))
>  {
> @@ -650,6 +653,7 @@ static void sve_probe_vqs(DECLARE_BITMAP(map, SVE_VQ_MAX))
>  void __init sve_init_vq_map(void)
>  {
>  	sve_probe_vqs(sve_vq_map);
> +	bitmap_copy(sve_vq_partial_map, sve_vq_map, SVE_VQ_MAX);
>  }
>
>  /*
> @@ -658,24 +662,60 @@ void __init sve_init_vq_map(void)
>   */
>  void sve_update_vq_map(void)
>  {
> -	sve_probe_vqs(sve_secondary_vq_map);
> -	bitmap_and(sve_vq_map, sve_vq_map, sve_secondary_vq_map, SVE_VQ_MAX);
> +	sve_probe_vqs(sve_tmp_vq_map);
> +	bitmap_and(sve_vq_map, sve_vq_map, sve_tmp_vq_map,
> +		   SVE_VQ_MAX);
> +	bitmap_or(sve_vq_partial_map, sve_vq_partial_map, sve_tmp_vq_map,
> +		  SVE_VQ_MAX);
>  }

I'm not quite following what's going on here. This is tracking both the
vector lengths available on all CPUs and the ones available on at least
one CPU? This raises a some questions:

  - do such franken-machines exist or are expected to exit?
  - how do we ensure this is always upto date?
  - what happens when we hotplug a new CPU with less available VQ?

>
>  /* Check whether the current CPU supports all VQs in the committed set */
>  int sve_verify_vq_map(void)
>  {
> -	int ret = 0;
> +	int ret = -EINVAL;
> +	unsigned long b;
>
> -	sve_probe_vqs(sve_secondary_vq_map);
> -	bitmap_andnot(sve_secondary_vq_map, sve_vq_map, sve_secondary_vq_map,
> -		      SVE_VQ_MAX);
> -	if (!bitmap_empty(sve_secondary_vq_map, SVE_VQ_MAX)) {
> +	sve_probe_vqs(sve_tmp_vq_map);
> +
> +	bitmap_complement(sve_tmp_vq_map, sve_tmp_vq_map, SVE_VQ_MAX);
> +	if (bitmap_intersects(sve_tmp_vq_map, sve_vq_map, SVE_VQ_MAX)) {
>  		pr_warn("SVE: cpu%d: Required vector length(s) missing\n",
>  			smp_processor_id());
> -		ret = -EINVAL;
> +		goto error;

The use of goto seems a little premature considering we don't have any
clean-up to do.

> +	}
> +
> +	if (!IS_ENABLED(CONFIG_KVM) || !is_hyp_mode_available())
> +		goto ok;
> +
> +	/*
> +	 * For KVM, it is necessary to ensure that this CPU doesn't
> +	 * support any vector length that guests may have probed as
> +	 * unsupported.
> +	 */
> +
> +	/* Recover the set of supported VQs: */
> +	bitmap_complement(sve_tmp_vq_map, sve_tmp_vq_map, SVE_VQ_MAX);
> +	/* Find VQs supported that are not globally supported: */
> +	bitmap_andnot(sve_tmp_vq_map, sve_tmp_vq_map, sve_vq_map, SVE_VQ_MAX);
> +
> +	/* Find the lowest such VQ, if any: */
> +	b = find_last_bit(sve_tmp_vq_map, SVE_VQ_MAX);
> +	if (b >= SVE_VQ_MAX)
> +		goto ok; /* no mismatches */
> +
> +	/*
> +	 * Mismatches above sve_max_virtualisable_vl are fine, since
> +	 * no guest is allowed to configure ZCR_EL2.LEN to exceed this:
> +	 */
> +	if (sve_vl_from_vq(bit_to_vq(b)) <= sve_max_virtualisable_vl) {
> +		pr_warn("SVE: cpu%d: Unsupported vector length(s) present\n",
> +			smp_processor_id());
> +		goto error;
>  	}
>
> +ok:
> +	ret = 0;
> +error:
>  	return ret;
>  }
>
> @@ -743,6 +783,7 @@ u64 read_zcr_features(void)
>  void __init sve_setup(void)
>  {
>  	u64 zcr;
> +	unsigned long b;
>
>  	if (!system_supports_sve())
>  		return;
> @@ -771,11 +812,31 @@ void __init sve_setup(void)
>  	 */
>  	sve_default_vl = find_supported_vector_length(64);
>
> +	bitmap_andnot(sve_tmp_vq_map, sve_vq_partial_map, sve_vq_map,
> +		      SVE_VQ_MAX);
> +
> +	b = find_last_bit(sve_tmp_vq_map, SVE_VQ_MAX);
> +	if (b >= SVE_VQ_MAX)
> +		/* No non-virtualisable VLs found */
> +		sve_max_virtualisable_vl = SVE_VQ_MAX;
> +	else if (WARN_ON(b == SVE_VQ_MAX - 1))
> +		/* No virtualisable VLs?  This is architecturally forbidden. */
> +		sve_max_virtualisable_vl = SVE_VQ_MIN;
> +	else /* b + 1 < SVE_VQ_MAX */
> +		sve_max_virtualisable_vl = sve_vl_from_vq(bit_to_vq(b + 1));
> +
> +	if (sve_max_virtualisable_vl > sve_max_vl)
> +		sve_max_virtualisable_vl = sve_max_vl;
> +
>  	pr_info("SVE: maximum available vector length %u bytes per vector\n",
>  		sve_max_vl);
>  	pr_info("SVE: default vector length %u bytes per vector\n",
>  		sve_default_vl);
>
> +	/* KVM decides whether to support mismatched systems. Just warn here: */
> +	if (sve_max_virtualisable_vl < sve_max_vl)
> +		pr_info("SVE: unvirtualisable vector lengths present\n");
> +
>  	sve_efi_setup();
>  }


--
Alex Bennée
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