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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 07/04/17 10:53, Yu Zhang 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. :-)
> 
>>
>>> @@ -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.

Yes, that's Jan's point -- that for EPT, there is effectively ASSERT()
that 2M and 1G entries are not p2m_ioreq_server; but for SVM, because of
the code duplication, there is not.

ASSERT()s are:
1. There to double-check that the assumptions you're making (i.e., "2M
and 1G entries can never be of type p2m_ioreq_server") are valid
2. Only enabled when debug=y, and so are generally not a performance
consideration.

You're making an assumption, so an ASSERT is useful; and it's only a
one-line check that will be removed for non-debug builds, so the
performance is not a consideration.

 -George


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