Re: [RFC PATCH 5/6] x86/alternatives: use temporary mm for text poking

[Andy Lutomirski]

On Wed, Aug 29, 2018 at 2:28 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Aug 29, 2018 at 01:11:46AM -0700, Nadav Amit wrote:

>> +     pte_clear(poking_mm, poking_addr, ptep);
>> +
>> +     /*
>> +      * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
>> +      * as it also flushes the corresponding "user" address spaces, which
>> +      * does not exist.
>> +      *
>> +      * Poking, however, is already very inefficient since it does not try to
>> +      * batch updates, so we ignore this problem for the time being.
>> +      *
>> +      * Since the PTEs do not exist in other kernel address-spaces, we do
>> +      * not use __flush_tlb_one_kernel(), which when PTI is on would cause
>> +      * more unwarranted TLB flushes.
>> +      */
>
> yuck :-), but yeah.

I'm sure we covered this ad nauseum when PTI was being developed, but
we were kind of in a rush, so:

Why do we do INVPCID at all?  The fallback path for non-INVPCID
systems uses invalidate_user_asid(), which should be faster than the
invpcid path.  And doesn't do a redundant flush in this case.

Can we just drop the INVPCID?  While we're at it, we could drop
X86_FEATURE_INVPCID_SINGLE entirely, since that's the only user.

--Andy