Re: [PATCH v4 09/13] nEPT: Add nEPT violation/misconfigration support

[Paolo Bonzini <pbonzini@redhat.com>]

Il 29/07/2013 18:24, Gleb Natapov ha scritto:
> On Mon, Jul 29, 2013 at 04:12:33PM +0200, Paolo Bonzini wrote:
>> Il 29/07/2013 15:20, Gleb Natapov ha scritto:
>>>> 2) in cases like this you just do not use likely/unlikely; the branch
>>>> will be very unlikely in the beginning, and very likely once shadow
>>>> pages are filled or in the no-EPT case.  Just let the branch predictor
>>>> adjust, it will probably do better than boolean tricks.
>>>>
>>> likely/unlikely are usually useless anyway. If you can avoid if()
>>> altogether this is a win since there is no branch to predict.
>>
>> However, if the branches are dynamically well-predicted,
>>
>>    if (simple)
>>        ...
>>    if (complex)
>>        ...
>>
>> is likely faster than
>>
>>    if (simple | complex)
>>
>> because the branches then are very very cheap, and it pays off to not
>> always evaluate the complex branch.
>
> Good point about about "|" always evaluating both. Is this the case
> with if (simple !=0 | complex != 0) too where theoretically compiler may
> see that if simple !=0 is true no need to evaluate the second one?

Yes (only if complex doesn't have any side effects, which is the case here).

>> Yeah, I also thought of always checking bad_mt_xwr and even using it to
>> subsume the present check too, i.e. turning it into
>> is_rsvd_bits_set_or_nonpresent.  It checks the same bits that are used
>> in the present check (well, a superset).  You can then check for
>> presence separately if you care, which you don't in
>> prefetch_invalid_gpte.  It requires small changes in the callers but
>> nothing major.
> 
> I do not get what is_rsvd_bits_set_or_nonpresent() will check exactly
> and why do we needed it, there are two places where we check
> present/reserved and in one of them we need to know which one it is.

You can OR bad_mt_xwr with 0x5555555555555555ULL (I think).  Then your
implementation of is_rsvd_bits_set() using bad_mt_xwr will return true
in all cases where the pte is non-present.  You can then call
is_present_pte to discriminate the two cases.

    if (is_rsvd_bits_set_or_nonpresent) {
        if (!present)
            ...
        else
            ...
    }

In more abstract terms this is:

    if (simple)
        ...
    if (complex)
        ...

to

    if (simple_or_complex) {
        if (simple)
            ...
        else
            ...
    }

This can actually make sense if simple is almost always false, because
then you save something from not evaluating it on the fast path.

But in this case, adding bad_mt_xwr to the non-EPT case is a small loss.

> Anyway order of checks in prefetch_invalid_gpte() is not relevant to
> that patchset, so lets better leave it to a separate discussion.

Yes.

Paolo

>>
>> But it still seems to me that we're in the above "if (simple ||
>> complex)" case and having a separate "if (!present)" check will be faster.
>>
>> Paolo
> 
> --
> 			Gleb.
>