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

[Peter Zijlstra]

On Mon, Jun 15, 2020 at 11:12:59AM -0700, Fenghua Yu 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?
> 
> My concern is TIF flags are precious (only 3 slots available). Defining
> a new TIF flag may be not worth it while rdmsr() can check if PASID
> is valid in the MSR. And performance here might not be a big issue
> in #GP.
> 
> But if you think using TIF flag is better, I can define a new TIF flag
> and maintain it per thread (init 0 when clone()/fork(), set 1 in fixup()).
> Then we can avoid using rdmsr() to check valid PASID in the MSR.

WHY ?!?! What do you need a TIF flag for?