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

[Peter Zijlstra]

On Mon, Jun 15, 2020 at 11:19:21AM -0700, Raj, Ashok wrote:
> On Mon, Jun 15, 2020 at 06:03:57PM +0200, Peter Zijlstra wrote:
> > 
> > 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?
> 
> The reason to check the rdmsr() is because we are using a hueristic taking
> GP faults. If we already setup the MSR, but we get it a second time it
> means the reason is something other than PASID_MSR not being set.
> 
> Ideally we should use the TIF_ to track this would be cheaper, but we were
> told those bits aren't easy to give out. 

Why do you need a TIF flag? Why not any other random flag in current?