Re: [PATCH v6 12/12] x86/traps: Fix up invalid PASID

[Andy Lutomirski <luto@kernel.org>]

On Mon, Jul 13, 2020 at 4:48 PM Fenghua Yu <fenghua.yu@intel.com> wrote:
>
> A #GP fault is generated when ENQCMD instruction is executed without
> a valid PASID value programmed in the current thread's PASID MSR. The
> #GP fault handler will initialize the MSR if a PASID has been allocated
> for this process.

Let's take a step back here.  Why are we trying to avoid IPIs?  If you
call munmap(), you IPI other CPUs running tasks in the current mm.  If
you do perf_event_open() and thus acquire RDPMC permission, you IPI
other CPUs running tasks in the current mm.  If you call modify_ldt(),
you IPI other CPUs running tasks in the current mm.  These events can
all happen more than once per process.

Now we have ENQCMD.  An mm can be assigned a PASID *once* in the model
that these patches support.  Why not just send an IPI using
essentially identical code to the LDT sync or the CR4.PCE sync?

--Andy