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

[Andy Lutomirski <luto@kernel.org>]

On Tue, Aug 27, 2019 at 4:55 PM Nadav Amit <namit@vmware.com> wrote:
>
> > 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.
>

We do this anyway if we don't have INVPCID_SINGLE, so it doesn't seem
so bad to also do it if there's a freed page table.