On Thu, 2023-07-27 at 09:16 -0500, Haitao Huang wrote:
On Wed, 26 Jul 2023 21:50:02 -0500, Huang, Kai kai.huang@intel.com wrote:
On Wed, 2023-07-26 at 18:02 -0700, Haitao Huang wrote:
Under heavy load, the SGX EPC reclaimer (ksgxd) may reclaim the SECS EPC
If I read correctly, Dave suggested to not use "high" (heavy in this sentence) or "low" pressure:
https://lore.kernel.org/lkml/op.179a4xs0wjvjmi@hhuan26-mobl.amr.corp.intel.c...
And I agree. For instance, consider this happens to one extremely "small" enclave, while there's a new "big" enclave starts to run. I don't think we should say this is "under heavy load". Just stick to the fact that the reclaimer may reclaim the SECS page.
Mybe I have some confusion here but I did not think Dave had issues with 'heavy load'. When this happens, the last page causing #PF (page A below) should be the the "youngest" in PTE and it got paged out together with the SECS before the #PF is even handled. Based on that the ksgxd moves 'young' pages to the back of the queue for reclaiming, for that to happen, almost all EPC pages must be paged out for all enclaves at that time, so it means heavy load to me. And that's also consistent with my tests.
I already provided an example: swapping out an "extreme small" enclave.
Anyway, no big deal to me.
page for an enclave and set encl->secs.epc_page to NULL. But the SECS EPC page is used for EAUG in the SGX page fault handler without checking for NULL and reloading.
Fix this by checking if SECS is loaded before EAUG and loading it if it was reclaimed.
The SECS page holds global enclave metadata. It can only be reclaimed when there are no other enclave pages remaining. At that point, virtually nothing can be done with the enclave until the SECS page is paged back in.
...
But it is still possible for a #PF for a non-SECS page to race with paging out the SECS page: when the last resident non-SECS page A triggers a #PF in a non-resident page B, and then page A and the SECS both are paged out before the #PF on B is handled.
Hitting this bug requires that race triggered with a #PF for EAUG.
The above race can happen for the normal ELDU path too, thus I suppose it will be better to mention why the normal ELDU path doesn't have this issue: it already does what this fix does.
Should we focus on the bug and fix itself instead of explaining a non-bug case? And the simple changes in this patch clearly show that too if people look for that.
So you spent a lot of text explaining the race condition, but such race condition applies to both ELDU and EAUG. I personally went to see the code whether ELDU has such issue too, and it turned out only EAUG has issue. If you mention this in the changelog perhaps I wouldn't need to go to read the code.
Anyway, just my 2cents.
And I don't want to let those block this patch, so feel free to add my tag:
Reviewed-by: Kai Huang kai.huang@intel.com