[PATCH 2/2] docs: riscv: hwprobe: Clarify misaligned keys are values not bitmasks

[Andrew Jones]

On Tue, May 21, 2024 at 11:36:06AM GMT, Evan Green wrote:
> On Sat, May 18, 2024 at 9:00 AM Yangyu Chen <cyy at cyyself.name> wrote:
> >
> > The original documentation says hwprobe keys are bitmasks, but actually,
> > they are values. This patch clarifies this to avoid confusion.
> >
> > Signed-off-by: Yangyu Chen <cyy at cyyself.name>
> 
> Hm, we also have this problem in the code, since
> hwprobe_key_is_bitmask() returns true for KEY_CPUPERF_0. This results
> in wrong information being returned for queries using the WHICH_CPU
> flag. If usermode asked for the set of CPUs that was specifically SLOW
> or EMULATED, the returned cpuset would also include cpus that were
> FAST. I believe all other queries are okay.
> 
> The one-liner fix is to just not return true for that key in
> hwprobe_key_is_bitmask(). But that's technically user-visible: if some
> software relied on the buggy behavior of FAST cpus being swept up in
> the query for SLOW or EMULATED cpus, this change would expose that.
> The grownups-eat-their-vegetables thing to do would be to define a new
> key that returns this same value, but doesn't return true in
> hwprobe_key_is_bitmask(). What do people think?

Even though I actually enjoy eating vegetables, I think it's unlikely
that we need to be so cautious for this. I feel like kernel updates
provide a bit of freedom to change results of hardware query syscalls,
even when run on the same hardware. Particularly the EMULATED query,
which I guess could change with a firmware update. And, even the SLOW
query could change if the probing was modified directly or indirectly.
IOW, applications that use the which-cpus syscall shouldn't freak out
if they don't get the same cpuset after a kernel update, which means
we can drop the FAST cpus from the result.

Thanks,
drew

> 
> -Evan
> 
> > ---
> >  Documentation/arch/riscv/hwprobe.rst | 31 ++++++++++++++++------------
> >  1 file changed, 18 insertions(+), 13 deletions(-)
> >
> > diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> > index 239be63f5089..4abfa3f9fe44 100644
> > --- a/Documentation/arch/riscv/hwprobe.rst
> > +++ b/Documentation/arch/riscv/hwprobe.rst
> > @@ -188,25 +188,30 @@ The following keys are defined:
> >         manual starting from commit 95cf1f9 ("Add changes requested by Ved
> >         during signoff")
> >
> > -* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance
> > +* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A value that contains performance
> >    information about the selected set of processors.
> >
> > -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned
> > -    scalar accesses is unknown.
> > +  * :c:macro:`RISCV_HWPROBE_MISALIGNED_MASK`: The bitmask of the misaligned
> > +    access performance field in the value of key `RISCV_HWPROBE_KEY_CPUPERF_0`.
> >
> > -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_EMULATED`: Misaligned scalar accesses are
> > -    emulated via software, either in or below the kernel.  These accesses are
> > -    always extremely slow.
> > +    The following values (not bitmasks) in this field are defined:
> >
> > -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned scalar accesses are
> > -    slower than equivalent byte accesses.  Misaligned accesses may be supported
> > -    directly in hardware, or trapped and emulated by software.
> > +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned
> > +      scalar accesses is unknown.
> >
> > -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned scalar accesses are
> > -    faster than equivalent byte accesses.
> > +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_EMULATED`: Misaligned scalar accesses are
> > +      emulated via software, either in or below the kernel.  These accesses are
> > +      always extremely slow.
> >
> > -  * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNSUPPORTED`: Misaligned scalar accesses
> > -    are not supported at all and will generate a misaligned address fault.
> > +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned scalar accesses are
> > +      slower than equivalent byte accesses.  Misaligned accesses may be supported
> > +      directly in hardware, or trapped and emulated by software.
> > +
> > +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned scalar accesses are
> > +      faster than equivalent byte accesses.
> > +
> > +    * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNSUPPORTED`: Misaligned scalar accesses
> > +      are not supported at all and will generate a misaligned address fault.
> >
> >  * :c:macro:`RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`: An unsigned int which
> >    represents the size of the Zicboz block in bytes.
> > --
> > 2.43.0
> >