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 11:28, Xia, Hongyan wrote:
> On Wed, 2019-12-11 at 11:10 +0000, Julien Grall wrote:
> > Hi Hongyan,
> > ...
> >
> > While this involves more instructions, how often do we expect the
> > code
> > to be called?
> >
> > Cheers,
> I don't expect this to be called very often in the current Xen.
> Although with direct map removal, a lot of the memory allocations
> (mostly xenheap allocations) will be mapped and unmapped on-demand and
> there is a much higher change of merging/shattering.
Thank you for the explanation. In order to merge/shatter, you need the 
buddy allocator to ensure all the xenheap are allocated contiguously. I 
am pretty unconvinced there are a lot of page allocated via xenheap. So 
the chance to have contiguous xenheap allocation is very limited.
But the merging/shattering can be counterproductive. An example short 
memory allocation (we have a few places like that):
   xmalloc(...);
   do something
   xfree(...);

We would end up to merge and then a few ms later shatter again. So it 
feels to me, that merging is probably not worth it (I am planning to 
discuss with Andrew today about it).
> However, the series moved all PTEs from xenheap to domheap, and we
> might see other things moved to domheap in the future, so we might not
> have many things left on xenheap anyway.
Typesafe is an important part of making our code base more secure than 
basic C (such as not mixing type).
In this case, I think if we start to merge/shatter a lot, then we have a 
bigger problem and we may want to consider to remove it (see above) So 
it feels the optimization is not worth it.
Note that I am not maintaining this code, so the final call is on Andrew 
and Jan.
Cheers,

--

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