Re: [PATCH v2 10/10] x86/mm: Try to preserve old TLB entries using PCID

[Nadav Amit]

Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Sat, Jun 17, 2017 at 11:26 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>>> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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 Is this commit message outdated?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 donât 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?