On Tue, 18 Jul 2023 15:56:27 -0500, Dave Hansen dave.hansen@intel.com wrote:
On 7/18/23 13:32, Haitao Huang wrote: ...
Ignore VA pages for now. Say for a system with 10 page EPC, 2 enclaves, each needs 5 pages non-SECS so total demand would be 12 pages. The ksgxd would only need to swap out 2 pages at the most to get one enclave fully loaded with 6 pages, and the other one with 4 pages. There is no chance the ksgxd would swap any one of two SECS pages.
We would need at least one enclave A of 10 pages total to squeeze out the other B completely. For that to happen B pretty much has to be sleeping all the time so the LRU based reclaiming would hit it but not pages of A. So no chance to hit #PF on pages of B still.
So some minimal pressure is needed to ensure SECS swapped. The higher the pressure the higher the chance to hit #PF while SECS is swapped.
What would the second-to-last non-SECS page be? A thread control page? VA page?
As long as *that* page can generate a page fault, then you only need two pages for this scenario to happen:
- Reclaimer takes encl->lock
- #PF occurs from another thread, blocks on encl->lock
- SECS is reclaimed
- encl->lock released
- #PF sees reclaimed SECS
I agree this is the race. But for this to happen, that is at #1 you have only one non-SECS page left so #3 can happen. That means it is already high pressure because reclaimer has swapped all other non-SECS. In my example of two enclaves of 5 non-EPC pages. #3 won't happen because you don't reach #1 with only one non-SECS left.
Thanks Haitao