Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes

[Nadav Amit <namit@vmware.com>]

> On Aug 27, 2019, at 4:13 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Fri, Aug 23, 2019 at 11:13 PM Nadav Amit <namit@vmware.com> wrote:
>> INVPCID is considerably slower than INVLPG of a single PTE. Using it to
>> flush the user page-tables when PTI is enabled therefore introduces
>> significant overhead.
>> 
>> Instead, unless page-tables are released, it is possible to defer the
>> flushing of the user page-tables until the time the code returns to
>> userspace. These page tables are not in use, so deferring them is not a
>> security hazard.
> 
> I agree and, in fact, I argued against ever using INVPCID in the
> original PTI code.
> 
> However, I don't see what freeing page tables has to do with this.  If
> the CPU can actually do speculative page walks based on the contents
> of non-current-PCID TLB entries, then we have major problems, since we
> don't actively flush the TLB for non-running mms at all.

That was not my concern.

> 
> I suppose that, if we free a page table, then we can't activate the
> PCID by writing to CR3 before flushing things.  But we can still defer
> the flush and just set the flush bit when we write to CR3.

This was my concern. I can change the behavior so the code would flush the
whole TLB instead. I just tried not to change the existing behavior too
much.