Re: [Xen-devel] [PATCH] x86: reduce Meltdown band-aid IPI overhead

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/01/18 12:07, Jan Beulich wrote:
> In case we can detect single-threaded guest processes (by checking
> whether we can account for all root page table uses locally on the vCPU
> that's running), there's no point in issuing a sync IPI upon an L4 entry
> update, as no other vCPU of the guest will have that page table loaded.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> This will apply cleanly only on top of all of the previously posted
> follow-ups to the Meltdown band-aid, but it wouldn't be difficult to
> move it ahead of some or all of them.
>
> On my test system, this improves kernel build times only 0.5...1%, but
> the effect may well be bigger on larger systems. But of course there's
> no improvement expected at heavily multi-threaded guests/processes.
>
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -3683,8 +3683,18 @@ long do_mmu_update(
>                          break;
>                      rc = mod_l4_entry(va, l4e_from_intpte(req.val), mfn,
>                                        cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
> -                    if ( !rc )
> -                        sync_guest = !cpu_has_no_xpti;
> +                    /*
> +                     * No need to sync if all uses of the page can be 
> accounted
> +                     * to the lock we hold, its pinned status, and uses on 
> this
> +                     * (v)CPU.
> +                     */
> +                    if ( !rc && !cpu_has_no_xpti &&
> +                         ((page->u.inuse.type_info & PGT_count_mask) >
> +                          (1 + !!(page->u.inuse.type_info & PGT_pinned) +
> +                           (pagetable_get_pfn(curr->arch.guest_table) == 
> mfn) +
> +                           (pagetable_get_pfn(curr->arch.guest_table_user) ==
> +                            mfn))) )

This unfortunately looks rather fragile.

Where does the 1 + come from in the beginning, and doesn't this go wrong
when guest_table == guest_table_user ?

At the very lease, it would probably be better to make a
count_local_references() helper to split the two pieces of logic.

~Andrew

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