> 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Is this commit message outdated? NR_DYNAMIC_ASIDS is set to 6.
More importantly, I do not see PCID 0 as reserved:
> +static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
> + u16 *new_asid, bool *need_flush)
> +{
>
[snip]
> + if (*new_asid >= NR_DYNAMIC_ASIDS) {
> + *new_asid = 0;
> + this_cpu_write(cpu_tlbstate.next_asid, 1);
> + }
> + *need_flush = true;
> +}
Am I missing something?
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