[PATCH v2 12/12] x86/traps: Fix up invalid PASID
ashok.raj at intel.com
Mon Jun 15 18:19:21 UTC 2020
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.
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