Re: [Xen-devel] [PATCH v2 1/2] x86/mm: factor out the code for shattering an l3 PTE

[Julien Grall <jgrall@xxxxxxxxxx>]

Hi Hongyan,

On 11/12/2019 10:28, Xia, Hongyan wrote:
> On Tue, 2019-12-10 at 16:20 +0100, Jan Beulich wrote:
> >          ol2e = l2e_from_intpte(
> >                     l2e_get_intpte(ol2e) + (PAGE_SIZE <<
> > PAGETABLE_ORDER));
> >
> > Of course, as mentioned before, I'm not overly happy to see type
> > safety lost in case like this one, where it's not needed like e.g.
> > further up to convert from L3 to L2 entry.
> Okay, so I did a comparison between the efficiency of the assembly
> under a release build.
>
> The old "type-safe" way requires 16 instructions to prepare the first
> l2e, and each iteration of the inner loop of populating l2t requires 7
> instructions.
>
> The new type-unsafe way requires 6 to prepare the first l2e, and each
> iteration of populating l2t takes 5 instructions.
>
> So the difference of populating l2t is 3600 vs. 2566 instructions,
> which is not very small.
While this involves more instructions, how often do we expect the code 
to be called?
Cheers,

--
Julien Grall

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