Re: xen_exit_mmap() questions

[Boris Ostrovsky]

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