Re: [RFC PATCH 03/13] x86/cpu: Enumerate User Interrupts support

[Sohil Mehta]

On 9/23/2021 3:24 PM, Thomas Gleixner wrote:
> On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
> > SENDUIPI is a special ring-3 instruction that makes a supervisor mode
> > memory access to the UPID and UITT memory. Currently, KPTI needs to be
> > off for User IPIs to work.  Processors that support user interrupts are
> > not affected by Meltdown so the auto mode of KPTI will default to off.
> >
> > Users who want to force enable KPTI will need to wait for a later
> > version of this patch series that is compatible with KPTI. We need to
> > allocate the UPID and UITT structures from a special memory region that
> > has supervisor access but it is mapped into userspace. The plan is to
> > implement a mechanism similar to LDT.
> Seriously?

Are questioning why we should add KPTI support if the hardware is not 
affected by Meltdown?

or

Why use an LDT like mechanism to do this?

I have listed this as one of the opens in the cover letter as well. I am 
not sure if users who force enable PTI would really care about User 
Interrupts.

Any input here would be helpful.

> > +	if (!cpu_feature_enabled(X86_FEATURE_UINTR))
> > +		goto disable_uintr;
> > +
> > +	/* checks the current processor's cpuid bits: */
> > +	if (!cpu_has(c, X86_FEATURE_UINTR))
> > +		goto disable_uintr;
> > +
> > +	/*
> > +	 * User Interrupts currently doesn't support PTI. For processors that
> > +	 * support User interrupts PTI in auto mode will default to off.  Need
> > +	 * this check only for users who have force enabled PTI.
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_PTI)) {
> > +		pr_info_once("x86: User Interrupts (UINTR) not enabled. Please disable PTI using 'nopti' kernel parameter\n");
> That message does not make sense. The admin has explicitly added 'pti'
> to the kernel command line on a CPU which is not affected. So why would
> he now have to add 'nopti' ?

Yup. I'll fix this and other issues in this patch.

I thought the user should know why UINTR has been disabled. In 
hindsight, this would have been better covered in the sample Readme or 
something similar.


Thanks,

Sohil