Messages in this thread | | | Subject | Re: xen_exit_mmap() questions | From | Boris Ostrovsky <> | Date | Wed, 26 Apr 2017 20:55:55 -0400 |
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On 04/26/2017 06:49 PM, Andy Lutomirski wrote: > On Wed, Apr 26, 2017 at 3:45 PM, Boris Ostrovsky > <boris.ostrovsky@oracle.com> 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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