RDTSCP | IA32_TSC_AUX on x86-32

RDTSCP | IA32_TSC_AUX on x86-32

[Sebastian Andrzej Siewior]

Hi,

the RDTSCP opcode is translated as "Read Time-Stamp Counter and
Processor ID". It does not read the Processor ID is merely returns the
value of IA32_TSC_AUX which has been previously written by the OS. This
is documented but is misleading if you never made it past the headline.

Now. On 64bit there is __vdso_getcpu and IA32_TSC_AUX is setup (it also
contains the node number but…) for its purpose.
On 32bit there is no __vdso_getcpu and IA32_TSC_AUX is not setup.
RDTSCP is not privileged and returns the proper timestamp but the "CPU"
field is always zero.

Is the format OS specific and not for user purpose or could IA32_TSC_AUX
be setup on 32bit?

Sebastian

Re: RDTSCP | IA32_TSC_AUX on x86-32

[Borislav Petkov]

On Wed, Nov 23, 2022 at 10:46:07AM +0100, Sebastian Andrzej Siewior wrote:
> Is the format OS specific and not for user purpose or could IA32_TSC_AUX
> be setup on 32bit?

I don't see why not.

I mean, we wanna remove 32-bit but if you wanna still improve it...

:-)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: RDTSCP | IA32_TSC_AUX on x86-32

[Sebastian Andrzej Siewior]

On 2022-11-23 10:59:29 [+0100], Borislav Petkov wrote:
> On Wed, Nov 23, 2022 at 10:46:07AM +0100, Sebastian Andrzej Siewior wrote:
> > Is the format OS specific and not for user purpose or could IA32_TSC_AUX
> > be setup on 32bit?
> 
> I don't see why not.
> 
> I mean, we wanna remove 32-bit but if you wanna still improve it...
> 
> :-)

Do we have a schedule for the removal? I mean we talk about it for
years…

Sebastian

Re: RDTSCP | IA32_TSC_AUX on x86-32

[Borislav Petkov]

On Wed, Nov 23, 2022 at 11:02:52AM +0100, Sebastian Andrzej Siewior wrote:
> Do we have a schedule for the removal? I mean we talk about it for
> years…

Hahah, nah, no schedule. I don't think the time has come yet - even if
we want to. I was mostly joking. :-)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: RDTSCP | IA32_TSC_AUX on x86-32

[Pavel Machek]

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On Wed 2022-11-23 11:23:36, Borislav Petkov wrote:
> On Wed, Nov 23, 2022 at 11:02:52AM +0100, Sebastian Andrzej Siewior wrote:
> > Do we have a schedule for the removal? I mean we talk about it for
> > years…
> 
> Hahah, nah, no schedule. I don't think the time has come yet - even if
> we want to. I was mostly joking. :-)

Given that Pentium-class machines are last x86s without serious
security holes, I'd keep 32-bit support for a while :-).

							Pavel
-- 
People of Russia, stop Putin before his war on Ukraine escalates.

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