Re: [RFC PATCH 3/5] RISC-V: hwprobe: Introduce which-cpus flag

Palmer Dabbelt
2023-09-25T11:23:44Z

On Thu, 21 Sep 2023 05:55:22 PDT (-0700), ajones@ventanamicro.com wrote:
> Introduce the first flag for the hwprobe syscall. The flag basically
> reverses its behavior, i.e. instead of populating the values of keys
> for a given set of cpus, the set of cpus after the call is the result
> of finding a set which supports the values of the keys. In order to
> do this, we implement pair merge and pair compare functions which
> take the type of value (a single value vs. a bitmap of booleans) into
> consideration. The flow for the which-cpus syscall variant is as
> follows:
>
> 1. Merge pairs into a set of pairs with unique keys
> 2. If any unknown keys are seen, return an empty set of cpus
> 3. If the platform is homogeneous, then check all the pairs
> against the "all cpu" values and return early
> 4. Otherwise, check all the pairs against each cpu individually

IIRC we talked about this in the patchwork call at some point, but IMO
this feature makes sense and you weren't the first person to bring up
having something like this. So I think at a high level it's completely
reasonable, some implementation comments follow.

>
> Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> ---
> Documentation/riscv/hwprobe.rst | 16 ++-
> arch/riscv/include/uapi/asm/hwprobe.h | 3 +
> arch/riscv/kernel/sys_riscv.c | 148 +++++++++++++++++++++++++-
> 3 files changed, 163 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> index 132e9acaa8f4..97b1e97e7dd2 100644
> --- a/Documentation/riscv/hwprobe.rst
> +++ b/Documentation/riscv/hwprobe.rst
> @@ -25,8 +25,20 @@ arch, impl), the returned value will only be valid if all CPUs in the given set
> have the same value. Otherwise -1 will be returned. For boolean-like keys, the
> value returned will be a logical AND of the values for the specified CPUs.
> Usermode can supply NULL for ``cpus`` and 0 for ``cpusetsize`` as a shortcut for
> -all online CPUs. There are currently no flags, this value must be zero for
> -future compatibility.
> +all online CPUs. The currently supported flags are:
> +
> +* :c:macro:`RISCV_HWPROBE_WHICH_CPUS`: This flag basically reverses the behavior
> + of sys_riscv_hwprobe(). Instead of populating the values of keys for a given
> + set of CPUs, the set of CPUs is initially all unset and the values of each key
> + are given. Upon return, the CPUs which all match each of the given key-value
> + pairs are set in ``cpus``. How matching is done depends on the key type. For

Kind of a meta-comment here, but we should add the key type to each key
number in the docs. I suppose maybe it should be obvious which is which
from the meaning of the keys, but can't hurt to be explicit about it.

> + value-like keys, matching means to be the exact same as the value. For
> + boolean-like keys, matching means the result of a logical AND of the pair's
> + value with the CPU's value is exactly the same as the pair's value. ``cpus``
> + may also initially have set bits, in which case the bits of any CPUs which do
> + not match the pairs will be cleared, but no other bits will be set.
> +
> +All other flags are reserved for future compatibility and must be zero.
>
> On success 0 is returned, on failure a negative error code is returned.
>
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index 86d08a0e617b..36683307c3e4 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -40,4 +40,7 @@ struct riscv_hwprobe {
> #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE 6
> /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
>
> +/* Flags */
> +#define RISCV_HWPROBE_WHICH_CPUS (1 << 0)
> +
> #endif
> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> index 14b6dfaa5d9f..c70a72fe6aee 100644
> --- a/arch/riscv/kernel/sys_riscv.c
> +++ b/arch/riscv/kernel/sys_riscv.c
> @@ -245,14 +245,145 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> }
> }
>
> +static bool hwprobe_key_is_map(__s64 key)
> +{
> + switch (key) {
> + case RISCV_HWPROBE_KEY_BASE_BEHAVIOR:
> + case RISCV_HWPROBE_KEY_IMA_EXT_0:
> + case RISCV_HWPROBE_KEY_CPUPERF_0:
> + return true;
> + }
> +
> + return false;
> +}
> +
> +static int hwprobe_pair_merge(struct riscv_hwprobe *to,
> + struct riscv_hwprobe *from)
> +{
> + if (to->key != from->key)
> + return -EINVAL;
> +
> + if (hwprobe_key_is_map(to->key)) {
> + to->value |= from->value;
> + return 0;
> + }
> +
> + return to->value == from->value ? 0 : -EINVAL;
> +}
> +
> +static bool hwprobe_pair_cmp(struct riscv_hwprobe *pair,
> + struct riscv_hwprobe *other_pair)
> +{
> + if (pair->key != other_pair->key)
> + return false;
> +
> + if (hwprobe_key_is_map(pair->key))
> + return (pair->value & other_pair->value) == other_pair->value;
> +
> + return pair->value == other_pair->value;
> +}
> +
> +static int hwprobe_which_cpus(struct riscv_hwprobe __user *pairs_user,
> + size_t pair_count, size_t cpusetsize,
> + cpumask_t *cpus)
> +{
> + struct riscv_hwprobe pairs[RISCV_HWPROBE_MAX_KEY + 1] = {
> + [0 ... RISCV_HWPROBE_MAX_KEY] = (struct riscv_hwprobe){ .key = -1 }
> + };
> + struct riscv_hwprobe pair;
> + struct vdso_data *vd = __arch_get_k_vdso_data();
> + struct arch_vdso_data *avd = &vd->arch_data;
> + bool clear_all = false;
> + cpumask_t one_cpu;
> + int cpu, ret;
> + size_t i;
> +
> + for (i = 0; i < pair_count; i++) {
> + ret = copy_from_user(&pair, &pairs_user[i], sizeof(pair));
> + if (ret)
> + return -EFAULT;
> +
> + if (pair.key >= 0 && pair.key <= RISCV_HWPROBE_MAX_KEY) {
> + if (pairs[pair.key].key == -1) {
> + pairs[pair.key] = pair;
> + } else {
> + ret = hwprobe_pair_merge(&pairs[pair.key], &pair);
> + if (ret)
> + return ret;
> + }
> + } else {
> + pair.key = -1;
> + pair.value = 0;
> + ret = copy_to_user(&pairs_user[i], &pair, sizeof(pair));
> + if (ret)
> + return -EFAULT;
> + clear_all = true;
> + }
> + }
> +
> + if (clear_all) {
> + cpumask_clear(cpus);
> + return 0;
> + }
> +
> + if (avd->homogeneous_cpus) {
> + for (i = 0; i <= RISCV_HWPROBE_MAX_KEY; i++) {
> + if (pairs[i].key == -1)
> + continue;
> +
> + pair.key = pairs[i].key;
> + pair.value = avd->all_cpu_hwprobe_values[pairs[i].key];
> +
> + if (!hwprobe_pair_cmp(&pair, &pairs[i])) {
> + cpumask_clear(cpus);
> + return 0;
> + }
> + }
> +
> + return 0;
> + }
> +
> + cpumask_clear(&one_cpu);
> +
> + for_each_cpu(cpu, cpus) {
> + cpumask_set_cpu(cpu, &one_cpu);
> +
> + for (i = 0; i <= RISCV_HWPROBE_MAX_KEY; i++) {
> + if (pairs[i].key == -1)
> + continue;
> +
> + pair.key = pairs[i].key;
> + pair.value = 0;
> + hwprobe_one_pair(&pair, &one_cpu);
> +
> + if (!hwprobe_pair_cmp(&pair, &pairs[i])) {
> + cpumask_clear_cpu(cpu, cpus);
> + break;
> + }
> + }
> +
> + cpumask_clear_cpu(cpu, &one_cpu);
> + }
> +
> + return 0;
> +}
> +
> static int do_riscv_hwprobe(struct riscv_hwprobe __user *pairs,
> size_t pair_count, size_t cpusetsize,
> unsigned long __user *cpus_user,
> unsigned int flags)
> {
> + bool which_cpus = false;
> + cpumask_t cpus;
> size_t out;
> int ret;
> - cpumask_t cpus;
> +
> + if (flags & RISCV_HWPROBE_WHICH_CPUS) {
> + if (!cpusetsize || !cpus_user)
> + return -EINVAL;
> + flags &= ~RISCV_HWPROBE_WHICH_CPUS;
> + which_cpus = true;
> + }
>
> /* Check the reserved flags. */
> if (flags != 0)
> @@ -274,11 +405,24 @@ static int do_riscv_hwprobe(struct riscv_hwprobe __user *pairs,
> if (ret)
> return -EFAULT;
>
> + cpumask_and(&cpus, &cpus, cpu_online_mask);
> +
> + if (which_cpus) {
> + if (cpumask_empty(&cpus))
> + cpumask_copy(&cpus, cpu_online_mask);
> + ret = hwprobe_which_cpus(pairs, pair_count, cpusetsize, &cpus);
> + if (ret)
> + return ret;
> + ret = copy_to_user(cpus_user, &cpus, cpusetsize);
> + if (ret)
> + return -EFAULT;
> + return 0;
> + }
> +
> /*
> * Userspace must provide at least one online CPU, without that
> * there's no way to define what is supported.
> */
> - cpumask_and(&cpus, &cpus, cpu_online_mask);
> if (cpumask_empty(&cpus))
> return -EINVAL;
> }