Re: [PATCH v7 07/27] arm64/sve: Check SVE virtualisability

Andrew Jones
2019-04-04T21:21:17Z

On Fri, Mar 29, 2019 at 01:00:32PM +0000, Dave Martin wrote:
> 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>
> Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
>
> ---
>
> QUESTION: The final structure of this code makes it quite natural to
> clamp the vector length for KVM guests to the maximum value supportable
> across all CPUs; such a value is guaranteed to exist, but may be
> surprisingly small on a given hardware platform.
>
> Should we be stricter and actually refuse to support KVM at all on such
> hardware? This may help to force people to fix Linux (or the
> architecture) if/when this issue comes up.

Blocking KVM would be too harsh, since the users of the host may not
care about guests with SVE, but still care about guests.

>
> For now, I stick with pr_warn() and make do with a limited SVE vector
> length for guests.

I think that's probably the best we can do.

>
> Changes since v5:
>
> * pr_info() about the presence of unvirtualisable vector lengths in
> sve_setup() upgrade to pr_warn(), for consistency with
> sve_verify_vq_map().
> ---
> arch/arm64/include/asm/fpsimd.h | 1 +
> arch/arm64/kernel/cpufeature.c | 2 +-
> arch/arm64/kernel/fpsimd.c | 86 ++++++++++++++++++++++++++++++++++-------
> 3 files changed, 73 insertions(+), 16 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 4061de1..7f8cc51 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1863,7 +1863,7 @@ 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 f59ea67..b219796a 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 */
> @@ -623,12 +629,6 @@ 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);
> -
> static void sve_probe_vqs(DECLARE_BITMAP(map, SVE_VQ_MAX))
> {
> unsigned int vq, vl;
> @@ -654,6 +654,7 @@ 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);
> }
>
> /*
> @@ -663,8 +664,11 @@ 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);
> + DECLARE_BITMAP(tmp_map, SVE_VQ_MAX);
> +
> + sve_probe_vqs(tmp_map);
> + bitmap_and(sve_vq_map, sve_vq_map, tmp_map, SVE_VQ_MAX);
> + bitmap_or(sve_vq_partial_map, sve_vq_partial_map, tmp_map, SVE_VQ_MAX);
> }
>
> /*
> @@ -673,18 +677,48 @@ void sve_update_vq_map(void)
> */
> int sve_verify_vq_map(void)
> {
> - int ret = 0;
> + DECLARE_BITMAP(tmp_map, SVE_VQ_MAX);
> + 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(tmp_map);
> +
> + bitmap_complement(tmp_map, tmp_map, SVE_VQ_MAX);
> + if (bitmap_intersects(tmp_map, sve_vq_map, SVE_VQ_MAX)) {
> pr_warn("SVE: cpu%d: Required vector length(s) missing\n",
> smp_processor_id());
> - ret = -EINVAL;
> + return -EINVAL;
> }
>
> - return ret;
> + if (!IS_ENABLED(CONFIG_KVM) || !is_hyp_mode_available())
> + return 0;
> +
> + /*
> + * 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(tmp_map, tmp_map, SVE_VQ_MAX);
> + /* Find VQs supported that are not globally supported: */
> + bitmap_andnot(tmp_map, tmp_map, sve_vq_map, SVE_VQ_MAX);
> +
> + /* Find the lowest such VQ, if any: */
> + b = find_last_bit(tmp_map, SVE_VQ_MAX);
> + if (b >= SVE_VQ_MAX)
> + return 0; /* 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());
> + return -EINVAL;
> + }
> +
> + return 0;
> }
>
> static void __init sve_efi_setup(void)
> @@ -751,6 +785,8 @@ u64 read_zcr_features(void)
> void __init sve_setup(void)
> {
> u64 zcr;
> + DECLARE_BITMAP(tmp_map, SVE_VQ_MAX);
> + unsigned long b;
>
> if (!system_supports_sve())
> return;
> @@ -779,11 +815,31 @@ void __init sve_setup(void)
> */
> sve_default_vl = find_supported_vector_length(64);
>
> + bitmap_andnot(tmp_map, sve_vq_partial_map, sve_vq_map,
> + SVE_VQ_MAX);
> +
> + b = find_last_bit(tmp_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_warn("SVE: unvirtualisable vector lengths present\n");
> +
> sve_efi_setup();
> }
>
> --
> 2.1.4
>

Reviewed-by: Andrew Jones <drjones@redhat.com>