RE: [PATCH 2/4] x86/sgx: Change counter sgx_nr_free_pages -> sgx_nr_used_pages

[Reshetova, Elena]

> On Mon, Mar 24, 2025 at 12:19:37PM +0000, Reshetova, Elena wrote:
> >
> > > On Fri, Mar 21, 2025 at 02:34:41PM +0200, Elena Reshetova wrote:
> > > > sgx_nr_free_pages is an atomic that is used to keep track of
> > > > free EPC pages and detect whenever page reclaiming should start.
> > > > Since successful execution of ENCLS[EUPDATESVN] requires empty
> > > > EPC and a fast way of checking for this, change this variable
> > > > around to indicate number of used pages instead. The subsequent
> > > > patch that introduces ENCLS[EUPDATESVN] will take use of this change.
> > >
> > > s/subsequent patch//
> >
> > Ok
> >
> > >
> > > You should rather express how EUPDATESVN trigger will depend on the
> > > state of sgx_nr_used_pages and sgx_nr_free_pages.
> >
> > How about this explanation:
> >
> > "By counting the # of used pages instead of #of free pages, it allows the
> > EPC page allocation path execute without a need to take the lock in all
> > but a single case when the first page is being allocated in EPC. This is
> > achieved via a fast check in atomic_long_inc_not_zero."
> 
> Yep, whole a lot more sense.
> 
> >
> > Also, if you think that it is hard to interpret the patch 2/4 without 4/4
> > I can also squeeze them together and then it becomes right away clear
> > why the change was done.
> >
> >
> > >
> > > >
> > > > No functional changes intended.
> > >
> > > Not really understanding how I should interpret this sentence.
> >
> > Just as usual: this patch itself doesn’t bring any functional changes
> > to the way as current SGX code works. I only needed this change to
> > implement patch 4/4 in more lockless way.
> >
> > >
> > > The commit message does not mention sgx_nr_used_pages, and neiher it
> > > makes a case why implementing the feature based on sgx_nr_free_pages
> is
> > > not possible.
> >
> > It is possible to implement it, in fact I did exactly this in the beginning
> instead,
> > but as mentioned previously this would have resulted in taking a lock for
> each
> > case the page is being allocated.
> 
> Have you benchmarked this (memory barrier vs putting the whole thing
> inside spinlock)?
> 
> I have doubts that this would even show in margins given how much e.g.,
> ELDU takes.

No, I haven’t benchmarked this. The reason to choose this approach (vs. the
other one I showed in email now) was purely logical - less locks and better code.

Best Regards,
Elena.