Re: [PATCH v2 2/6] RISC-V: Add a syscall for HW probing

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Feb 09, 2023 at 09:09:16AM -0800, Evan Green wrote:
> On Mon, Feb 6, 2023 at 10:32 PM Conor Dooley <conor@kernel.org> wrote:
> >
> > Hey Evan, Greg,
> >
> >
> > On 7 February 2023 06:13:39 GMT, Greg KH <gregkh@linuxfoundation.org> wrote:
> > >On Mon, Feb 06, 2023 at 12:14:51PM -0800, Evan Green wrote:
> > >> We don't have enough space for these all in ELF_HWCAP{,2} and there's no
> > >> system call that quite does this, so let's just provide an arch-specific
> > >> one to probe for hardware capabilities.  This currently just provides
> > >> m{arch,imp,vendor}id, but with the key-value pairs we can pass more in
> > >> the future.
> > >
> > >Ick, this is exactly what sysfs is designed to export in a sane way.
> > >Why not just use that instead?  The "key" would be the filename, and the
> > >value the value read from the filename.  If the key is not present, the
> > >file is not present and it's obvious what is happening, no fancy parsing
> > >and ABI issues at all.
> >
> > https://lore.kernel.org/linux-riscv/20221201160614.xpomlqq2fzpzfmcm@kamzik/
> >
> > This is the sysfs interface that I mentioned drew
> > suggested on the v1.
> > I think it fits ~perfectly with what Greg is suggesting too.
> 
> Whoops, I'll admit I missed that comment when I reviewed the feedback
> from v1. I spent some time thinking about sysfs. The problem is this
> interface will be needed in places like very early program startup. If
> we're trying to use this in places like the ifunc selector to decide
> which memcpy to use, having to go open and read a fistful of files is
> going to be complex that early, and rough on performance.

How is it going to be any different on "performance" than a syscall?  Or
complex?  It should be almost identical overall as this is all in-ram
and not any real I/o is happening.  You are limited only by the speed of
your cpu.

> Really this is data that would go great in the aux vector, except
> there's probably too much of it to justify preparing and copying into
> every new process. You could point the aux vector into a vDSO data
> area. This has the advantage of great performance and no syscall, but
> has the disadvantages of making that data ABI, and requiring it all to
> be known up front (eg the kernel can't compute any answers on the
> fly).
> 
> After discussions with Palmer, my plan for the next version is to move
> this into a vDSO function plus a syscall. Private vDSO data will be
> prepped with common answers for the "all CPUs" case, avoiding the need
> for a syscall in most cases and making this fast. Since the data is
> hidden behind the vdso function, it's not ABI, which is a plus. Then
> the vdso function can fall back to the syscall for cases with exotic
> CPU masks or keys that are unknown/expensive to compute at runtime.

I still think that's wrong, as you are wanting a set of key/values here,
which is exactly what sysfs is designed for.

Please benchmark this first.  Heck, if you don't like the
open/read/close syscall overhead, use my readfile() syscall patch that I
keep proposing every 6 months or so to remove that overhead.  That would
be a good reason to get that code accepted finally :)

thanks,

greg k-h