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. Cheers, Ashok