Re: [PATCH] altp2m: Allow the hostp2m to be shared

[George Dunlap <george.dunlap@citrix.com>]

On 27/04/16 16:18, Tamas K Lengyel wrote:
> On Wed, Apr 27, 2016 at 9:01 AM, George Dunlap <george.dunlap@citrix.com>
> wrote:
> 
>> On 21/04/16 18:10, Tamas K Lengyel wrote:
>>> Don't propagate altp2m changes from ept_set_entry for memshare as
>> memshare
>>> already has the lock. We call altp2m propagate changes once memshare
>>> successfully finishes. Also, allow the hostp2m entries to be of type
>>> p2m_ram_shared.
>>>
>>> Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
>>
>> Sorry for the delay in reviewing -- trying to get my head back around
>> the altp2m code.  On the whole looks reasonable, but one question...
>>
>>> ---
>>> Cc: George Dunlap <george.dunlap@eu.citrix.com>
>>> Cc: Keir Fraser <keir@xen.org>
>>> Cc: Jan Beulich <jbeulich@suse.com>
>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>> Cc: Jun Nakajima <jun.nakajima@intel.com>
>>> Cc: Kevin Tian <kevin.tian@intel.com>
>>> ---
>>>  xen/arch/x86/mm/mem_sharing.c | 11 +++++++++++
>>>  xen/arch/x86/mm/p2m-ept.c     |  2 +-
>>>  xen/arch/x86/mm/p2m.c         |  7 +++----
>>>  3 files changed, 15 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/mm/mem_sharing.c
>> b/xen/arch/x86/mm/mem_sharing.c
>>> index a522423..d5b4b2d 100644
>>> --- a/xen/arch/x86/mm/mem_sharing.c
>>> +++ b/xen/arch/x86/mm/mem_sharing.c
>>> @@ -35,6 +35,7 @@
>>>  #include <asm/p2m.h>
>>>  #include <asm/atomic.h>
>>>  #include <asm/event.h>
>>> +#include <asm/altp2m.h>
>>>  #include <xsm/xsm.h>
>>>
>>>  #include "mm-locks.h"
>>> @@ -1026,6 +1027,16 @@ int mem_sharing_share_pages(struct domain *sd,
>> unsigned long sgfn, shr_handle_t
>>>      /* We managed to free a domain page. */
>>>      atomic_dec(&nr_shared_mfns);
>>>      atomic_inc(&nr_saved_mfns);
>>> +
>>> +    if( altp2m_active(cd) )
>>> +    {
>>> +        p2m_access_t a;
>>> +        struct p2m_domain *p2m = p2m_get_hostp2m(cd);
>>> +        p2m->get_entry(p2m, cgfn, NULL, &a, 0, NULL, NULL);
>>> +        p2m_altp2m_propagate_change(cd, _gfn(cgfn), smfn, PAGE_ORDER_4K,
>>> +                                    p2m_ram_shared, a);
>>> +    }
>>> +
>>>      ret = 0;
>>>
>>>  err_out:
>>> diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
>>> index 3cb6868..1ac3018 100644
>>> --- a/xen/arch/x86/mm/p2m-ept.c
>>> +++ b/xen/arch/x86/mm/p2m-ept.c
>>> @@ -846,7 +846,7 @@ out:
>>>      if ( is_epte_present(&old_entry) )
>>>          ept_free_entry(p2m, &old_entry, target);
>>>
>>> -    if ( rc == 0 && p2m_is_hostp2m(p2m) )
>>> +    if ( rc == 0 && p2m_is_hostp2m(p2m) && p2mt != p2m_ram_shared )
>>>          p2m_altp2m_propagate_change(d, _gfn(gfn), mfn, order, p2mt,
>> p2ma);
>>>
>>>      return rc;
>>> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
>>> index b3fce1b..d2aebf7 100644
>>> --- a/xen/arch/x86/mm/p2m.c
>>> +++ b/xen/arch/x86/mm/p2m.c
>>> @@ -1739,11 +1739,10 @@ int p2m_set_altp2m_mem_access(struct domain *d,
>> struct p2m_domain *hp2m,
>>>      /* Check host p2m if no valid entry in alternate */
>>>      if ( !mfn_valid(mfn) )
>>>      {
>>> -        mfn = hp2m->get_entry(hp2m, gfn_l, &t, &old_a,
>>> -                              P2M_ALLOC | P2M_UNSHARE, &page_order,
>> NULL);
>>> +        mfn = hp2m->get_entry(hp2m, gfn_l, &t, &old_a, 0, &page_order,
>> NULL);
>>
>> Why are you getting rid of P2M_ALLOC here?  What happens if the hp2m
>> entry is populate-on-demand?
>>
> 
> There is a check further down here that only allows p2m_ram_rw and
> p2m_ram_shared. 

So what P2M_ALLOC means is, "If this is entry is PoD, then please
populate it so I get a ram page."  So the only way you can get a
p2m_populate_on_demand type returned is if you remove this flag.  Leave
it and (assuming there's enough ram to go around), you'll always get
p2m_ram_rw.  :-)

> On the non-altp2m path mem_access doesn't request P2M_ALLOC
> either (but doesn't check the type), so I would say mem_access is not
> compatible with PoD.

Off the top of my head I can't see a reason why they couldn't co-exist
in principle, if you added P2M_ALLOC in a few key places.

 -George


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