Re: [PATCH v3 1/2] x86/sgx: Use sgx_nr_used_pages for EPC page count instead of sgx_nr_free_pages

[Huang, Kai]

On Tue, 2025-04-15 at 14:51 +0300, 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

The mentioning of ENCLS[EUPDATESVN] is kinda out of blue here.  It's better to
introduce it first like the next patch does.

> EPC and preferably a fast lockless way of checking for this
> condition in all code paths where EPC is already used, change the
> reclaiming code to track the number of used pages via
> sgx_nr_used_pages instead of sgx_nr_free_pages.
> For this change to work in the page reclamation code, add a new
> variable, sgx_nr_total_pages, that will keep track of total
> number of EPC pages.
> 
> It would have been possible to implement ENCLS[EUPDATESVN] using
> existing sgx_nr_free_pages counter and a new sgx_nr_total_pages
> counter, but it won't be possible to avoid taking a lock *every time*
> a new EPC page is being allocated. The conversion of sgx_nr_free_pages
> into sgx_nr_used_pages allows avoiding the lock in all cases except
> when it is the first EPC page being allocated via a quick
> atomic_long_inc_not_zero check.
> 
> Note: The serialization for sgx_nr_total_pages is not needed because
> the variable is only updated during the initialization and there's no
> concurrent access.
> 
> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>

Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>