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

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

On 4/7/2017 5:40 PM, Jan Beulich wrote:
>>>> On 06.04.17 at 17:53, <yu.c.zhang@linux.intel.com> 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. :-)

>
>> @@ -816,6 +822,22 @@ ept_set_entry(struct p2m_domain *p2m, unsigned long gfn, mfn_t mfn,
>>           new_entry.suppress_ve = is_epte_valid(&old_entry) ?
>>                                       old_entry.suppress_ve : 1;
>>   
>> +    /*
>> +     * p2m_ioreq_server is only used for 4K pages, so the
>> +     * count shall only happen on ept page table entries.
>> +     */
>> +    if ( p2mt == p2m_ioreq_server )
>> +    {
>> +        ASSERT(i == 0);
>> +        p2m->ioreq.entry_count++;
>> +    }
>> +
>> +    if ( ept_entry->sa_p2mt == p2m_ioreq_server )
>> +    {
>> +        ASSERT(p2m->ioreq.entry_count > 0 && i == 0);
> I think this would better be two ASSERT()s, so if one triggers it's
> clear what problem it was right away. The two conditions aren't
> really related to one another.
>
>> @@ -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

> 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).
>
>> @@ -606,6 +615,8 @@ p2m_pt_set_entry(struct p2m_domain *p2m, unsigned long gfn, mfn_t mfn,
>>   
>>       if ( page_order == PAGE_ORDER_4K )
>>       {
>> +        p2m_type_t p2mt_old;
>> +
>>           rc = p2m_next_level(p2m, &table, &gfn_remainder, gfn,
>>                               L2_PAGETABLE_SHIFT - PAGE_SHIFT,
>>                               L2_PAGETABLE_ENTRIES, PGT_l1_page_table, 1);
>> @@ -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.

As to ept, since there's already a variable 'i', which may be greater 
than 0 - so I added an ASSERT.

Yu
> Jan
>


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