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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 15, 2020 at 08:48:54AM -0700, Fenghua Yu wrote:
> 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.

But it doesn't guarantee the PASID is the right one. Suppose both the mm
has a PASID and the MSR has a VALID one, but the MSR isn't the mm one.
Then we NO-OP. So if the exception was due to us having the wrong PASID,
we stuck.

> 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.

So I get that you want to set the PASID on-demand, but I find this all
really weird code to make that happen.

> > > +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.

That's not in fact what I meant, but yes, you can take exceptions while
!IF just fine.

> > > +	 * 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?

I don't get why you need a rdmsr here, or why not having one would
require a TIF flag. Is that because this MSR is XSAVE/XRSTOR managed?

> > > +	 */
> > > +	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);

How much more expensive is the wrmsr over the rdmsr? Can't we just
unconditionally write the current PASID and call it a day?