Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1631779

Re: xen_exit_mmap() questions

Show all headers | View raw

On 04/26/2017 04:52 PM, Andy Lutomirski wrote:
> I was trying to understand xen_drop_mm_ref() to update it for some
> changes I'm working on, and I'm wondering whether we need
> xen_exit_mmap() at all.
>
> AFAICS the intent is to force all CPUs to drop their lazy uses of the
> mm being destroyed so it can be unpinned before tearing down the page
> tables, thus making it faster to tear down the page tables.  This
> seems like it'll speed up xen_set_pud() and xen_set_pmd(), but this
> seems like it may be of rather limited value. 

Why do you think it's of limited value? Without it we will end up with a
hypercall for each update.

Or is your point that the number of those update is relatively small
when we are tearing down?


>  Could we get away with
> deleting it?
>
> Also, this code in drop_other_mm_ref() looks dubious to me:
>
>     /* If this cpu still has a stale cr3 reference, then make sure
>        it has been flushed. */
>     if (this_cpu_read(xen_current_cr3) == __pa(mm->pgd))
>         load_cr3(swapper_pg_dir);
>
> If cr3 hasn't been flushed to the hypervisor because we're in a lazy
> mode, why would load_cr3() help?  Shouldn't this be xen_mc_flush()
> instead?

load_cr3() actually ends with xen_mc_flush() by way of xen_write_cr3()
-> xen_mc_issue().

-boris

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

xen_exit_mmap() questions Andy Lutomirski <luto@kernel.org> - 2017-04-26 23:00 +0200
  Re: xen_exit_mmap() questions Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-04-27 00:50 +0200
    Re: xen_exit_mmap() questions Andy Lutomirski <luto@kernel.org> - 2017-04-27 01:00 +0200
      Re: xen_exit_mmap() questions Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-04-27 03:00 +0200
        Re: xen_exit_mmap() questions Andy Lutomirski <luto@kernel.org> - 2017-04-27 03:10 +0200
          Re: xen_exit_mmap() questions Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-04-27 15:30 +0200
            Re: xen_exit_mmap() questions Andy Lutomirski <luto@kernel.org> - 2017-04-27 18:50 +0200
              Re: xen_exit_mmap() questions Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-04-27 22:10 +0200
                Re: xen_exit_mmap() questions Andy Lutomirski <luto@amacapital.net> - 2017-04-27 23:30 +0200
        Re: xen_exit_mmap() questions Ingo Molnar <mingo@kernel.org> - 2017-04-27 08:30 +0200
        Re: [Xen-devel] xen_exit_mmap() questions "Jan Beulich" <JBeulich@suse.com> - 2017-04-27 09:50 +0200

csiph-web