Re: [PATCH v1 3/6] RISC-V: hwprobe: Introduce which-cpus flag

Evan Green
2023-10-12T17:41:17Z

On Wed, Oct 11, 2023 at 6:56 AM Andrew Jones <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 a pair compare function which takes the type of
> value (a single value vs. a bitmask of booleans) into consideration.
> We also implement vdso support for the new flag.
>
> Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> ---
> Documentation/riscv/hwprobe.rst | 16 ++++-
> arch/riscv/include/asm/hwprobe.h | 24 +++++++
> arch/riscv/include/uapi/asm/hwprobe.h | 3 +
> arch/riscv/kernel/sys_hwprobe.c | 93 +++++++++++++++++++++++++--
> arch/riscv/kernel/vdso/hwprobe.c | 68 +++++++++++++++++---
> 5 files changed, 190 insertions(+), 14 deletions(-)
>
> diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> index c57437e40ffb..576aa03f56bb 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

This isn't quite right anymore, I reckon. The cpuset passed in is used
as an initial set, and this function removes CPUs from the set that
have differing values from those given in the array of pairs. If an
empty cpuset is passed in, then the initial set used is all online
cpus.

> + 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/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 7cad513538d8..a68764149e51 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -15,4 +15,28 @@ static inline bool riscv_hwprobe_key_is_valid(__s64 key)
> return key >= 0 && key <= RISCV_HWPROBE_MAX_KEY;
> }
>
> +static inline bool hwprobe_key_is_bitmask(__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 inline bool riscv_hwprobe_pair_cmp(struct riscv_hwprobe *pair,
> + struct riscv_hwprobe *other_pair)
> +{
> + if (pair->key != other_pair->key)
> + return false;
> +
> + if (hwprobe_key_is_bitmask(pair->key))
> + return (pair->value & other_pair->value) == other_pair->value;
> +
> + return pair->value == other_pair->value;
> +}
> +
> #endif
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index 006bfb48343d..1d4134befc48 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -38,4 +38,7 @@ struct riscv_hwprobe {
> #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)
> /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
>
> +/* Flags */
> +#define RISCV_HWPROBE_WHICH_CPUS (1 << 0)
> +
> #endif
> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> index 69ad5f793374..de294538ca25 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -166,10 +166,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> }
> }
>
> -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)
> +static int hwprobe_get_values(struct riscv_hwprobe __user *pairs,
> + size_t pair_count, size_t cpusetsize,
> + unsigned long __user *cpus_user,
> + unsigned int flags)
> {
> size_t out;
> int ret;
> @@ -223,6 +223,91 @@ static int do_riscv_hwprobe(struct riscv_hwprobe __user *pairs,
> return 0;
> }
>
> +static int hwprobe_get_cpus(struct riscv_hwprobe __user *pairs,
> + size_t pair_count, size_t cpusetsize,
> + unsigned long __user *cpus_user,
> + unsigned int flags)
> +{
> + cpumask_t cpus, one_cpu;
> + bool clear_all = false;
> + size_t i;
> + int ret;
> +
> + if (flags != RISCV_HWPROBE_WHICH_CPUS)
> + return -EINVAL;

We'll have to be careful if we add another flag to deal with how it
behaves here too. I think the choice you made here is correct as it's
the most defensive. It's (usually) easy to change a failure to a
non-failure, but an ABI compatibility issue to change a
weird-but-nonfailing behavior to a different behavior. This might be
worth a comment, but I also tend to love comments more than most, so
up to you.

> +
> + if (!cpusetsize || !cpus_user)
> + return -EINVAL;
> +
> + if (cpusetsize > cpumask_size())
> + cpusetsize = cpumask_size();
> +
> + ret = copy_from_user(&cpus, cpus_user, cpusetsize);
> + if (ret)
> + return -EFAULT;
> +
> + cpumask_and(&cpus, &cpus, cpu_online_mask);
> + if (cpumask_empty(&cpus))
> + cpumask_copy(&cpus, cpu_online_mask);

I worry this is accident prone. If the caller asks for some set of
CPUs that don't happen to be online right now, they get an answer for
a completely different set of CPUs. I'm all for having a shorthand for
"all online CPUs", but I think it should be more explicit. If you
moved the cpumask_and() below the if(cpumask_empty()), then IMO it
would be a clean shorthand: pass in an empty set to get all online
CPUs.

> +
> + cpumask_clear(&one_cpu);
> +
> + for (i = 0; i < pair_count; i++) {
> + struct riscv_hwprobe pair, tmp;
> + int cpu;
> +
> + ret = copy_from_user(&pair, &pairs[i], sizeof(pair));
> + if (ret)
> + return -EFAULT;
> +
> + if (!riscv_hwprobe_key_is_valid(pair.key)) {
> + clear_all = true;
> + pair = (struct riscv_hwprobe){ .key = -1, };
> + ret = copy_to_user(&pairs[i], &pair, sizeof(pair));
> + if (ret)
> + return -EFAULT;
> + }
> +
> + if (clear_all)
> + continue;
> +
> + tmp = (struct riscv_hwprobe){ .key = pair.key, };
> +
> + for_each_cpu(cpu, &cpus) {
> + cpumask_set_cpu(cpu, &one_cpu);
> +
> + hwprobe_one_pair(&tmp, &one_cpu);
> +
> + if (!riscv_hwprobe_pair_cmp(&tmp, &pair))
> + cpumask_clear_cpu(cpu, &cpus);
> +
> + cpumask_clear_cpu(cpu, &one_cpu);
> + }
> + }
> +
> + if (clear_all)
> + cpumask_clear(&cpus);
> +
> + ret = copy_to_user(cpus_user, &cpus, cpusetsize);
> + if (ret)
> + return -EFAULT;
> +
> + 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)
> +{
> + if (flags & RISCV_HWPROBE_WHICH_CPUS)
> + return hwprobe_get_cpus(pairs, pair_count, cpusetsize,
> + cpus_user, flags);
> +
> + return hwprobe_get_values(pairs, pair_count, cpusetsize,
> + cpus_user, flags);
> +}
> +
> #ifdef CONFIG_MMU
>
> static int __init init_hwprobe_vdso_data(void)
> diff --git a/arch/riscv/kernel/vdso/hwprobe.c b/arch/riscv/kernel/vdso/hwprobe.c
> index 026b7645c5ab..e6c324d64544 100644
> --- a/arch/riscv/kernel/vdso/hwprobe.c
> +++ b/arch/riscv/kernel/vdso/hwprobe.c
> @@ -3,6 +3,7 @@
> * Copyright 2023 Rivos, Inc
> */
>
> +#include <linux/string.h>
> #include <linux/types.h>
> #include <vdso/datapage.h>
> #include <vdso/helpers.h>
> @@ -11,14 +12,9 @@ extern int riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
> size_t cpusetsize, unsigned long *cpus,
> unsigned int flags);
>
> -/* Add a prototype to avoid -Wmissing-prototypes warning. */
> -int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
> - size_t cpusetsize, unsigned long *cpus,
> - unsigned int flags);
> -
> -int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
> - size_t cpusetsize, unsigned long *cpus,
> - unsigned int flags)
> +static int riscv_vdso_get_values(struct riscv_hwprobe *pairs, size_t pair_count,
> + size_t cpusetsize, unsigned long *cpus,
> + unsigned int flags)
> {
> const struct vdso_data *vd = __arch_get_vdso_data();
> const struct arch_vdso_data *avd = &vd->arch_data;
> @@ -50,3 +46,59 @@ int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
>
> return 0;
> }
> +
> +static int riscv_vdso_get_cpus(struct riscv_hwprobe *pairs, size_t pair_count,
> + size_t cpusetsize, unsigned long *cpus,
> + unsigned int flags)
> +{
> + const struct vdso_data *vd = __arch_get_vdso_data();
> + const struct arch_vdso_data *avd = &vd->arch_data;
> + struct riscv_hwprobe *p = pairs;
> + struct riscv_hwprobe *end = pairs + pair_count;
> + bool clear_all = false;
> +
> + if (!cpusetsize || !cpus)
> + return -EINVAL;
> +
> + if (flags != RISCV_HWPROBE_WHICH_CPUS || !avd->homogeneous_cpus)
> + return riscv_hwprobe(pairs, pair_count, cpusetsize, cpus, flags);
> +

Hm, I realize now that doing the shorthand of "empty set == all online
cpus" leaves the VDSO in a slight lurch, as we now have to figure out
how to come up with the cpu_online_mask in usermode. It'd be a bummer
if the shorthand always bounced to the system call, as I think this
will be the most common case. Is there a way we could stash the
cpu_online_mask in the vDSO data without it going stale? Any other
ideas?

> + while (p < end) {
> + if (riscv_hwprobe_key_is_valid(p->key)) {
> + struct riscv_hwprobe t = {
> + .key = p->key,
> + .value = avd->all_cpu_hwprobe_values[p->key],
> + };
> +
> + if (!riscv_hwprobe_pair_cmp(&t, p))
> + clear_all = true;
> + } else {
> + clear_all = true;
> + p->key = -1;
> + p->value = 0;
> + }
> + p++;
> + }
> +
> + if (clear_all)
> + memset(cpus, 0, cpusetsize);
> +
> + return 0;
> +}
> +
> +/* Add a prototype to avoid -Wmissing-prototypes warning. */
> +int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
> + size_t cpusetsize, unsigned long *cpus,
> + unsigned int flags);
> +
> +int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
> + size_t cpusetsize, unsigned long *cpus,
> + unsigned int flags)
> +{
> + if (flags & RISCV_HWPROBE_WHICH_CPUS)
> + return riscv_vdso_get_cpus(pairs, pair_count, cpusetsize,
> + cpus, flags);
> +
> + return riscv_vdso_get_values(pairs, pair_count, cpusetsize,
> + cpus, flags);
> +}
> --
> 2.41.0