Re: [Xen-devel] xen_exit_mmap() questions

[Andy Lutomirski <luto@xxxxxxxxxx>]

On Wed, Apr 26, 2017 at 5:55 PM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
>
>
> 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).

But that's what leave_mm() does.  To be fair, the x86 lazy TLB
management is a big mess, and this came up because I'm trying to clean
it up without removing it.

I suppose I can try to keep xen_exit_mmap() working.  Is there a
simple way to try to unpin but to not treat it as an error if the
hypervisor rejects it?

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