Andy Lutomirski <l...@kernel.org> wrote: > On Sat, Jun 17, 2017 at 11:26 PM, Nadav Amit <nadav.a...@gmail.com> wrote: >>> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski <l...@kernel.org> wrote: >>> >>> PCID is a "process context ID" -- it's what other architectures call >>> an address space ID. Every non-global TLB entry is tagged with a >>> PCID, only TLB entries that match the currently selected PCID are >>> used, and we can switch PGDs without flushing the TLB. x86's >>> PCID is 12 bits. >>> >>> This is an unorthodox approach to using PCID. x86's PCID is far too >>> short to uniquely identify a process, and we can't even really >>> uniquely identify a running process because there are monster >>> systems with over 4096 CPUs. To make matters worse, past attempts >>> to use all 12 PCID bits have resulted in slowdowns instead of >>> speedups. >>> >>> This patch uses PCID differently. We use a PCID to identify a >>> recently-used mm on a per-cpu basis. An mm has no fixed PCID >>> binding at all; instead, we give it a fresh PCID each time it's >>> loaded except in cases where we want to preserve the TLB, in which >>> case we reuse a recent value. >>> >>> In particular, we use PCIDs 1-3 for recently-used mms and we reserve >>> PCID 0 for swapper_pg_dir and for PCID-unaware CR3 users (e.g. EFI). >>> Nothing ever switches to PCID 0 without flushing PCID 0 non-global >>> pages, so PCID 0 conflicts won't cause problems. >> >> Is this commit message outdated? > > Yes, it's old. Will fix.
Just to clarify: I asked since I don’t understand how the interaction with PCID-unaware CR3 users go. Specifically, IIUC, arch_efi_call_virt_teardown() can reload CR3 with an old PCID value. No?
