Re: [Xen-devel] xen_exit_mmap() questions

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/26/2017 06:49 PM, Andy Lutomirski wrote:
> On Wed, Apr 26, 2017 at 3:45 PM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
> > 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?
> The latter.  Also, unless I'm missing something, xen_set_pte() doesn't
> have the optimization.  I haven't looked at exactly how page table
> teardown works, but if it clears each PTE individually, then that's
> the bulk of the work.
>
> > >  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().
> xen_mc_issue() does:
>
>         if ((paravirt_get_lazy_mode() & mode) == 0)
>                 xen_mc_flush();
>
> I assume the load_cr3() is intended to deal with the case where we're
> in lazy mode, but we'll still be in lazy mode, right?  Or does it
> serve some other purpose?
Of course. I can't read (I ignored the "== 0" part).

Apparently the early version had an explicit flush but then it 
disappeared (commit 9f79991d4186089e228274196413572cc000143b).
The point of CR3 loading here, I believe, is to make sure the hypervisor 
knows that the (v)CPU is no longer using the the mm's cr3 (we are 
loading swapper_pgdir here).
-boris

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