Re: [PATCH] tools/power turbostat: Fix MSRs with CONFIG_MULTIUSER=n

[Calvin Owens <calvin@wbinvd.org>]

On Monday 06/16 at 01:30 +0000, Zhang, Rui wrote:
> On Fri, 2025-06-13 at 19:20 -0700, Calvin Owens wrote:
> > Handle ENOSYS from cap_get_proc() in check_for_cap_sys_rawio(), so
> > turbostat can display temperatures when running on kernels compiled
> > without multiuser support.
> > 
> > Signed-off-by: Calvin Owens <calvin@wbinvd.org>
> > ---
> >  tools/power/x86/turbostat/turbostat.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/power/x86/turbostat/turbostat.c
> > b/tools/power/x86/turbostat/turbostat.c
> > index 925556b90770..f7d665913a52 100644
> > --- a/tools/power/x86/turbostat/turbostat.c
> > +++ b/tools/power/x86/turbostat/turbostat.c
> > @@ -6496,8 +6496,13 @@ int check_for_cap_sys_rawio(void)
> >  	int ret = 0;
> >  
> >  	caps = cap_get_proc();
> > -	if (caps == NULL)
> > +	if (caps == NULL) {
> > +		/* Support CONFIG_MULTIUSER=n */
> > +		if (errno == ENOSYS)
> 
> Can you point me where this knowledge comes from?
> 
> I downloaded the libcap source and didn't see how ENOSYS is set.

Hi Rui,

When the kernel is built without multiuser support, the capget() et al.
syscalls are #ifdef'd out:

    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/capability.c#n32

...so userspace will get -ENOSYS when it tries to call them, and that
ends up being propagated to errno in userspace.

Admittedly it is sort of implicit. Maybe a better way to "fix" this
would be to warn the user if the capability check fails, but still
attempt to access the MSR devices? I can do that if you prefer.

That is my only problem here: when check_for_cap_sys_rawio() fails, the
current code doesn't attempt to access the MSR devices at all, even
though in my case it would actually work.

I realize this is very weird: it came up when I was recently including
turbostat as part of an extremely tiny bootable utility image.

Thanks,
Calvin

> thanks,
> rui
> > +			return 0;
> > +
> >  		return 1;
> > +	}
> >  
> >  	if (cap_get_flag(caps, CAP_SYS_RAWIO, CAP_EFFECTIVE,
> > &cap_flag_value)) {
> >  		ret = 1;
>