Message-ID: <tencent_C5BBD6F106D73F662084BD91904F54504805@qq.com>
Date: Thu, 30 May 2024 11:36:45 +0800
From: Yangyu Chen <cyy@...self.name>
To: Evan Green <evan@...osinc.com>
Cc: Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>,
 Andrew Jones <ajones@...tanamicro.com>,
 Andy Chiu <andy.chiu@...ive.com>,
 Clément Léger <cleger@...osinc.com>,
 Conor Dooley <conor.dooley@...rochip.com>,
 Costa Shulyupin <costa.shul@...hat.com>,
 Jonathan Corbet <corbet@....net>,
 Paul Walmsley <paul.walmsley@...ive.com>,
 Sami Tolvanen <samitolvanen@...gle.com>,
 linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 linux-riscv@...ts.infradead.org
Subject: Re: [PATCH] RISC-V: hwprobe: Add MISALIGNED_PERF key

On 2024/5/30 02:26, Evan Green wrote:
> RISCV_HWPROBE_KEY_CPUPERF_0 was mistakenly flagged as a bitmask in
> hwprobe_key_is_bitmask(), when in reality it was an enum value. This
> causes problems when used in conjunction with RISCV_HWPROBE_WHICH_CPUS,
> since SLOW, FAST, and EMULATED have values whose bits overlap with
> each other. If the caller asked for the set of CPUs that was SLOW or
> EMULATED, the returned set would also include CPUs that were FAST.
> 
> Introduce a new hwprobe key, RISCV_HWPROBE_KEY_MISALIGNED_PERF, which
> returns the same values in response to a direct query (with no flags),
> but is properly handled as an enumerated value. As a result, SLOW,
> FAST, and EMULATED are all correctly treated as distinct values under
> the new key when queried with the WHICH_CPUS flag.
> 
> Leave the old key in place to avoid disturbing applications which may
> have already come to rely on the broken behavior.
> 
> Fixes: e178bf146e4b ("RISC-V: hwprobe: Introduce which-cpus flag")
> Signed-off-by: Evan Green <evan@...osinc.com>
> 
> ---
> 
> 
> Note: Yangyu also has a fix out for this issue at [1]. That fix is much
> tidier, but comes with the slight risk that some very broken userspace
> application may break now that FAST cpus are not included for the query
> of which cpus are SLOW or EMULATED.

Indeed. Since the value of FAST is 0b11, the SLOW and EMULATED are 0b10 and
0b01 respectively.

When this key is treated as a bitmask and query with
RISCV_HWPROBE_WHICH_CPUS if a CPU has a superset bitmask of the requested
value on the requested key, it will remain in the CPU mask. Otherwise, the
CPU will be clear in the CPU mask. But when a key is treated as a value, we
will just do a comparison. if it is not equal, then the CPU will be clear
in the CPU. That's why FAST cpus are included when querying with SLOW or
EMULATED with RISCV_HWPROBE_KEY_CPUPERF_0 key now.

For me, deprecating the original hwprobe key and introducing a new key
would be a better solution than changing the behavior as my patch did.

> I wanted to get this fix out so that
> we have both as options, and can discuss. These fixes are mutually
> exclusive, don't take both.

It's better to note this strange behavior on
Documentation/arch/riscv/hwprobe.rst so users can quickly understand the
differences on the behavior of these two keys.

The C code part looks good to me.

> 
> [1] https://lore.kernel.org/linux-riscv/tencent_01F8E0050FB4B11CC170C3639E43F41A1709@qq.com/
> 
> ---
> Documentation/arch/riscv/hwprobe.rst | 8 ++++++--
> arch/riscv/include/asm/hwprobe.h | 2 +-
> arch/riscv/include/uapi/asm/hwprobe.h | 1 +
> arch/riscv/kernel/sys_hwprobe.c | 1 +
> 4 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> index 204cd4433af5..616ee372adaf 100644
> --- a/Documentation/arch/riscv/hwprobe.rst
> +++ b/Documentation/arch/riscv/hwprobe.rst
> @@ -192,8 +192,12 @@ The following keys are defined:
> supported as defined in the RISC-V ISA manual starting from commit
> d8ab5c78c207 ("Zihintpause is ratified").
> 
> -* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance
> - information about the selected set of processors.
> +* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: Deprecated. Returns similar values to
> + :c:macro:`RISCV_HWPROBE_KEY_MISALIGNED_PERF`, but the key was mistakenly
> + classified as a bitmask rather than a value.
> +
> +* :c:macro:`RISCV_HWPROBE_KEY_MISALIGNED_PERF`: An enum value describing the
> + performance of misaligned scalar accesses on the selected set of processors.
> 
> * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned
> accesses is unknown.
> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 630507dff5ea..150a9877b0af 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -8,7 +8,7 @@
> 
> #include <uapi/asm/hwprobe.h>
> 
> -#define RISCV_HWPROBE_MAX_KEY 6
> +#define RISCV_HWPROBE_MAX_KEY 7
> 
> static inline bool riscv_hwprobe_key_is_valid(__s64 key)
> {
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index dda76a05420b..bc34e33fef23 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -68,6 +68,7 @@ struct riscv_hwprobe {
> #define RISCV_HWPROBE_MISALIGNED_UNSUPPORTED (4 << 0)
> #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)
> #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE 6
> +#define RISCV_HWPROBE_KEY_MISALIGNED_PERF 7
> /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
> 
> /* Flags */
> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> index 969ef3d59dbe..c8b7d57eb55e 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -208,6 +208,7 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> break;
> 
> case RISCV_HWPROBE_KEY_CPUPERF_0:
> + case RISCV_HWPROBE_KEY_MISALIGNED_PERF:
> pair->value = hwprobe_misaligned(cpus);
> break;
> 

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