Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes
From: Nadav Amit
Date: Tue Aug 27 2019 - 19:55:08 EST
> On Aug 27, 2019, at 4:13 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Fri, Aug 23, 2019 at 11:13 PM Nadav Amit <namit@xxxxxxxxxx> 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