Re: [PATCH v4 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On 6/20/2016 9:38 PM, Jan Beulich wrote:
>>>> On 20.06.16 at 14:06, <yu.c.zhang@linux.intel.com> wrote:
>> Suppose resolve_misconfig() is modified to change all p2m_ioreq_server
>> entries(which also
>> have e.recalc flag turned on) back to p2m_ram_rw. And suppose we have
>> ioreq server 1, which
>> emulates gfn A, and ioreq server 2 which emulates gfn B:
>>
>> 1> At the beginning, ioreq server 1 is attached to p2m_ioreq_server, and
>> gfn A is write protected
>> by setting it to p2m_ioreq_server;
>>
>> 2> ioreq server 1 is detached from p2m_ioreq_server, left gfn A's p2m
>> type unchanged;
>>
>> 3> After the detachment of ioreq server 1,
>> p2m_change_entry_type_global() is called, all ept
>> entries are invalidated;
>>
>> 4> Later, ioreq server 2 is attached to p2m_ioreq_server;
>>
>> 5> Gfn B is set to p2m_ioreq_server, although its corresponding ept
>> entry was invalidated,
>> ept_set_entry() will trigger resolve_misconfig(), which will set the p2m
>> type of gfn B back to
>> p2m_ram_rw;
>>
>> 6> ept_set_entry() will set gfn B's p2m type to p2m_ioreq_server next;
>> And now, we have two
>> ept entries with p2m_ioreq_server type - gfn A's and gfn B's.
>>
>> With no live migration, things could work fine - later accesses to gfn A
>> will ultimately change
>> its type back to p2m_ram_rw.
>>
>> However, if live migration is started(all pte entries invalidated
>> again), resolve_misconfig() would
>> change both gfn A's and gfn B's p2m type back to p2m_ram_rw, which means
>> the emulation of
>> gfn B would fail.
> Why would it? Changes to p2m_ram_logdirty won't alter
> p2m_ioreq_server entries, and hence changes from it back to
> p2m_ram_rw won't either.

Oh, above example is based on the assumption that resolve_misconfig() is 
extended
to handle the p2m_ioreq_server case(see my "Suppose resolve_misconfig() 
is modified...").
The code change could be something like below:

@@ -542,10 +542,14 @@ static int resolve_misconfig(struct p2m_domain 
*p2m, unsigned long gfn)

-                    if ( e.recalc && p2m_is_changeable(e.sa_p2mt) )
+                   if ( e.recalc )
                      {
-                         e.sa_p2mt = p2m_is_logdirty_range(p2m, gfn + 
i, gfn + i)
-                                     ? p2m_ram_logdirty : p2m_ram_rw;
+                         if ( e.sa_p2mt == p2m_ioreq_server )
+                             e.sa_p2mt = p2m_ram_rw;
+                         else if ( p2m_is_changeable(e.sa_p2mt) )
+                             e.sa_p2mt = p2m_is_logdirty_range(p2m, gfn 
+ i, gfn + i)
+                                         ? p2m_ram_logdirty : p2m_ram_rw;
+
                           ept_p2m_type_to_flags(p2m, &e, e.sa_p2mt, 
e.access);
                      }
                      e.recalc = 0;

With changes like this, both p2m types of gfn A and gfn B from above example
would be set to p2m_ram_rw if log dirty is enabled.
So that's what I am worrying - if a user unintentionally typed "xl save" 
during
the emulation process , the emulation would fail. We can let the 
enable_logdirty()
return false if XenGT is detected, but we still wish to keep the log 
dirty feature.

>
> And then - didn't we mean to disable that part of XenGT during
> migration, i.e. temporarily accept the higher performance
> overhead without the p2m_ioreq_server entries? In which case
> flipping everything back to p2m_ram_rw after (completed or
> canceled) migration would be exactly what we want. The (new
> or previous) ioreq server should attach only afterwards, and
> can then freely re-establish any p2m_ioreq_server entries it
> deems necessary.
>

Well, I agree this part of XenGT should be disabled during migration. 
But in such
case I think it's device model's job to trigger the p2m type 
flipping(i.e. by calling
HVMOP_set_mem_type). And the device model should be notified first when the
migration begins - we may need new patches to do so if XenGT is going to 
support
vGPU migration in the future.

Thanks
Yu

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