Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 27 Aug 2019 17:30:55 -0700 | Subject | Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes |
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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.
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