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 6:22 PM, George Dunlap wrote:
> 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@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.
> 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.

Thanks George.
I do not worry about the cost of the ASSERT() itself, but the effort of 
peeking a p2m:
p2m_flags_to_type(l1e_get_flags(*p2m_entry));
And this cannot be  removed during runtime.

Yu

>   -George
>
>


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