Re: [Xen-devel] [PATCH v12 5/6] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.04.17 at 11:53, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> On 4/7/2017 5:40 PM, Jan Beulich wrote:
>>>>> On 06.04.17 at 17:53, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/mm/p2m-ept.c
>>> +++ b/xen/arch/x86/mm/p2m-ept.c
>>> @@ -544,6 +544,12 @@ static int resolve_misconfig(struct p2m_domain *p2m, 
>>> unsigned long gfn)
>>>                       e.ipat = ipat;
>>>                       if ( e.recalc && p2m_is_changeable(e.sa_p2mt) )
>>>                       {
>>> +                         if ( e.sa_p2mt == p2m_ioreq_server )
>>> +                         {
>>> +                             ASSERT(p2m->ioreq.entry_count > 0);
>>> +                             p2m->ioreq.entry_count--;
>>> +                         }
>>> +
>>>                            e.sa_p2mt = p2m_is_logdirty_range(p2m, gfn + i, 
>>> gfn + i)
>>>                                        ? p2m_ram_logdirty : p2m_ram_rw;
>> I don't think this can be right: Why would it be valid to change the
>> type from p2m_ioreq_server to p2m_ram_rw (or p2m_ram_logdirty)
>> here, without taking into account further information? This code
>> can run at any time, not just when you want to reset things. So at
>> the very least there is a check missing whether a suitable ioreq
>> server still exists (and only if it doesn't you want to do the type
>> reset).
> 
> Sorry, Jan. I think we have discussed this quite long ago.
> Indeed, there's information lacked here, and that's why global_logdirty 
> is disallowed
> when there's remaining p2m_ioreq_server entries. :-)

log-dirty isn't the interesting part (which is why I did put it in
parentheses). You change ioreq-server to ram-rw here
regardless of whether we're actually cleaning up after a
detached server.

>>> @@ -965,7 +987,7 @@ static mfn_t ept_get_entry(struct p2m_domain *p2m,
>>>       if ( is_epte_valid(ept_entry) )
>>>       {
>>>           if ( (recalc || ept_entry->recalc) &&
>>> -             p2m_is_changeable(ept_entry->sa_p2mt) )
>>> +             p2m_check_changeable(ept_entry->sa_p2mt) )
>> I think the distinction between these two is rather arbitrary, and I
>> also think this is part of the problem above: Distinguishing log-dirty
>> from ram-rw requires auxiliary data to be consulted. The same
>> ought to apply to ioreq-server, and then there wouldn't be a need
>> to have two p2m_*_changeable() flavors.
> 
> Well, I think we have also discussed this quite long ago, here is the link.
> https://lists.xen.org/archives/html/xen-devel/2016-09/msg01017.html 

That was a different discussion, namely not considering this ...

>> Of course the subsequent use p2m_is_logdirty_range() may then
>> need amending.
>>
>> In the end it looks like you have the inverse problem here compared
>> to above: You should return ram-rw when the reset was already
>> initiated. At least that's how I would see the logic to match up with
>> the log-dirty handling (where the _effective_ rather than the last
>> stored type is being returned).

... at all.

>>> @@ -629,6 +640,21 @@ p2m_pt_set_entry(struct p2m_domain *p2m, unsigned long 
>>> gfn, mfn_t mfn,
>>>           if ( entry_content.l1 != 0 )
>>>               p2m_add_iommu_flags(&entry_content, 0, iommu_pte_flags);
>>>   
>>> +        p2mt_old = p2m_flags_to_type(l1e_get_flags(*p2m_entry));
>>> +
>>> +        /*
>>> +         * p2m_ioreq_server is only used for 4K pages, so
>>> +         * the count shall only be performed for level 1 entries.
>>> +         */
>>> +        if ( p2mt == p2m_ioreq_server )
>>> +            p2m->ioreq.entry_count++;
>>> +
>>> +        if ( p2mt_old == p2m_ioreq_server )
>>> +        {
>>> +            ASSERT(p2m->ioreq.entry_count > 0);
>>> +            p2m->ioreq.entry_count--;
>>> +        }
>>> +
>>>           /* level 1 entry */
>>>           p2m->write_p2m_entry(p2m, gfn, p2m_entry, entry_content, 1);
>> I think to match up with EPT you also want to add
>>
>>      ASSERT(p2mt_old != p2m_ioreq_server);
>>
>> to the 2M and 1G paths.
> 
> Is this really necessary? 2M and 1G page does not have p2mt_old, 
> defining one and peek the p2m type just
> to have an ASSERT does not seem quite useful - and will hurt the 
> performance.

I don't follow. Matching up with the 4k case, what you'd add would
be

    ASSERT(p2m_flags_to_type(flags) != p2m_ioreq_server);

Jan

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