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Re: [Xen-devel] [PATCH] x86/EPT: adjustments for redundant function arguments
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Fri, 20 Dec 2019 14:58:19 +0000
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- Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Wei Liu <wl@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 20 Dec 2019 14:58:31 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 12/20/19 2:41 PM, Jan Beulich wrote:
> On 20.12.2019 15:26, George Dunlap wrote:
>> On 12/20/19 2:21 PM, Jan Beulich wrote:
>>> In ept_p2m_type_to_flags() passing in type and access as separate
>>> parameters can be considered an optimization, as all callers set the
>>> respective fields in the entry being updated before the call. Retain
>>> this behavior but add assertions.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> In what way is it an optimization?
>
> There's no pointer de-ref needed; the values will already come in
> via registers. And "can be considered" because possibly some
> compilers are smart enough to eliminate the pointer de-ref again
> (but then it'll still be a bitfield extract, which callers may
> be able to avoid).
Right; on the whole I'd rather let compilers do this sort of
micro-optimization, and only do this "manual" sort of optimization with
some sort of benchmarks showing that is has some kind of effect.
>
>> I don't necessarily oppose this, but given that 3 of the 4 callers
>> literally do something like:
>>
>> ept_p2m_type_to_flags(p2m, &e, e.sa_p2mt, e.access);
>>
>> It seems like just getting rid of the extraneous arguments might a be
>> better option.
>
> That was my original intention as well, but iirc Andrew didn't like
> it when we discussed it back then (the context here being XSA-304).
I did a quick skim through those threads and couldn't find any comment
on this issue. Could you point me to the mail with it? (Or Andy, would
you care to repeat your argument?)
Ultimately the patch as it stands is only making the existing code
safer, so I'm OK with giving it my Ack if you don't want to pursue the
other option; but I'd prefer trying to understand and potentially
improve things while we're at it. (And if there *is* a good reason for
passing in parallel parameters, it would be good to record it in a
comment so we don't have this conversation again in 3 years' time.)
- George
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