Re: [RFC PATCH v4 04/26] x86/sgx: Add SGX_CHILD_PRESENT hardware error code

Sean Christopherson
2021-02-09T17:08:14Z

On Tue, Feb 09, 2021, Dave Hansen wrote:
> On 2/9/21 8:48 AM, Sean Christopherson wrote:
> > On Tue, Feb 09, 2021, Dave Hansen wrote:
> >> On 2/8/21 2:54 AM, Kai Huang wrote:
> >> ...
> >>> Add SGX_CHILD_PRESENT for use by SGX virtualization to assert EREMOVE
> >>> failures are expected, but only due to SGX_CHILD_PRESENT.
> >> This paragraph broke my brain when I read it. How about:
> >>
> >> Add a definition of SGX_CHILD_PRESENT. It will be used
> >> exclusively by the SGX virtualization driver to suppress EREMOVE
> >> warnings.
> > Maybe worth clarifying that the driver isn't suppressing warnings willy-nilly?
> > And the error code isn't about suppressing warnings, it's about identifying the
> > expected EREMOVE failure scenario. The patch that creates the separate helper
> > for doing EREMOVE without the WARN is what provides the suppression mechanism.
> >
> > Something like this?
> >
> > Add a definition of SGX_CHILD_PRESENT. It will be used exclusively by
> > the SGX virtualization driver to handle recoverable EREMOVE errors when
> > saniziting EPC pages after they are reclaimed from a guest.
>
> Looks great to me. One nit: to a me, "reclaim" is different than
> "free". Reclaim is a specific operation where a page is taken from one
> user and reclaimed for other use. "Free" is the more general case
> (which includes reclaim) when a physical page is no longer being used
> (because the user is done *or* had the page reclaimed) and may be either
> used by someone else or put in a free pool.
>
> I *think* this is actually a "free" operation, rather than a "reclaim".
> IIRC, this code gets used at munmap().

It does. I used reclaim because userspace, which does the freeing from this
code's perspective, never touches the EPC pages. The SGX_CHILD_PRESENT case is
handling the scenario where userspace has for all intents and purposed reclaimed
the EPC from a guest. If the guest cleanly tears down its enclaves, EREMOVE
will not fail.

"free" is probably better though, the above is far from obvious and still not
guaranteed to be a true reclaim scenario. If using "freed", drop the "from a
guest" part.