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
>
> 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
> + 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;

So this is now essentailly two syscalls. IMO it'd be cleaner to split
out the implementations into two functions (ie,
hwprobe_{which_cpus,which_featurs}() or whatever) rather than have an
early out and the rest inline.

Also: maybe we want a whole hwprobe file? It's sort of its own thing
now, and it's only going to get bigger...

> + }
> +
> /*
> * 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;
> }