Re: [PATCH v2 12/12] x86/traps: Fix up invalid PASID

[Fenghua Yu]

Hi, Peter,
On Mon, Jun 15, 2020 at 09:56:49AM +0200, Peter Zijlstra wrote:
> On Fri, Jun 12, 2020 at 05:41:33PM -0700, Fenghua Yu wrote:
> > +/*
> > + * Apply some heuristics to see if the #GP fault was caused by a thread
> > + * that hasn't had the IA32_PASID MSR initialized.  If it looks like that
> > + * is the problem, try initializing the IA32_PASID MSR. If the heuristic
> > + * guesses incorrectly, take one more #GP fault.
> 
> How is that going to help? Aren't we then going to run this same
> heuristic again and again and again?

The heuristic always initializes the MSR with the per mm PASID IIF the
mm has a valid PASID but the MSR doesn't have one. This heuristic usually
happens only once on the first #GP in a thread.

If the next #GP still comes in, the heuristic finds out the MSR already
has a valid PASID and thus will not fixup the MSR any more. The fixup()
returns "false" and lets others to handle the #GP.

So the heuristic will be executed once (at most) and won't be executed
again and again.

> 
> > + */
> > +bool __fixup_pasid_exception(void)
> > +{
> > +	u64 pasid_msr;
> > +	unsigned int pasid;
> > +
> > +	/*
> > +	 * This function is called only when this #GP was triggered from user
> > +	 * space. So the mm cannot be NULL.
> > +	 */
> > +	pasid = current->mm->pasid;
> > +	/* If the mm doesn't have a valid PASID, then can't help. */
> > +	if (invalid_pasid(pasid))
> > +		return false;
> > +
> > +	/*
> > +	 * Since IRQ is disabled now, the current task still owns the FPU on
> 
> That's just weird and confusing. What you want to say is that you rely
> on the exception disabling the interrupt.

I checked SDM again. You are right. #GP can be interrupted by machine check
or other interrupts. So I cannot assume the current task still owns the FPU.
Instead of directly rdmsr() and wrmsr(), I will add helpers that can access
either the MSR on the processor or the PASID state in the memory.

> 
> > +	 * this CPU and the PASID MSR can be directly accessed.
> > +	 *
> > +	 * If the MSR has a valid PASID, the #GP must be for some other reason.
> > +	 *
> > +	 * If rdmsr() is really a performance issue, a TIF_ flag may be
> > +	 * added to check if the thread has a valid PASID instead of rdmsr().
> 
> I don't understand any of this. Nobody except us writes to this MSR, we
> should bloody well know what's in it. What gives?

Patch 4 describes how to manage the MSR and patch 7 describes the format
of the MSR (20-bit PASID value and bit 31 is valid bit).

Are they sufficient to help? Or do you mean something else?

> > +	 */
> > +	rdmsrl(MSR_IA32_PASID, pasid_msr);
> > +	if (pasid_msr & MSR_IA32_PASID_VALID)
> > +		return false;
> > +
> > +	/* Fix up the MSR if the MSR doesn't have a valid PASID. */
> > +	wrmsrl(MSR_IA32_PASID, pasid | MSR_IA32_PASID_VALID);
> > +
> > +	return true;
> > +}
> > -- 
> > 2.19.1
> > 

Thanks.

-Fenghua