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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 29/07/2013 14:34, Gleb Natapov ha scritto:
>>>> But I think what you _really_ want is not avoiding conditional branches.
>>> The idea is that it is hard for branch prediction to predict correct
>>> result when correct result depends on guest's page table that can
>>> contain anything, so in some places shadow paging code uses boolean
>>> logic to avoid branches, in this case it is hard to avoid if() anyway
>>> since the function invocation is in the if().
>>
>> Yes, I get the idea, but is_rsvd_bits_set should be predicted unlikely,
>> no?  If the guest has to run, it must use mostly valid ptes. :)
>>
> You see, you are confused and you want branch prediction not to be? :)
> If your guest is KVM is_rsvd_bits_set() will be likely much more then
> unlikely because KVM misconfigures EPT entries to cache MMIO addresses,
> so all the "unlikely" cases will be fixed by shadow pages and will not
> reappear (until shadow pages are zapped), but misconfigured entries will
> continue to produces violations.

But then:

1) MMIO is a slow path anyway, losing 10 cycles on a mispredicted branch
is not going to help much.  Fast page faults are all I would optimize for.

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.

>> Especially if you change prefetch_invalid_gpte to do the reserved bits
>> test after the present test (so that is_rsvd_bits_set is only called on
>> present pagetables), is_rsvd_bits_set's result should be really
>> well-predicted. 
> Nope, for ept page tables present is not a single bit, it is three bits
> which by themselves can have invalid values.

We're not checking the validity of the bits in the is_present_gpte test,
we're checking it in the is_rsvd_bits_set test (is_present_gpte is doing
just "(pte & 7) != 0").  It doesn't change anything in the outcome of
prefetch_invalid_gpte, and it makes the ordering consistent with
walk_addr_generic which already tests presence before reserved bits.

So doing this swap should be a win anyway.

>>                   At this point (and especially since function invocation
>> is always in "if"s), using boolean logic to avoid branches does not make
>> much sense anymore for this function.
> 
> That's true.

So are you going to change to "if"s?

Paolo