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Date:   Thu, 30 Aug 2018 01:45:29 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>
CC:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "suresh.b.siddha@...el.com" <suresh.b.siddha@...el.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: RE: [PATCH v13 10/13] x86/sgx: Add sgx_einit() for initializing
 enclaves

> > > > > Some kind of counter is required to keep track of the power cycle.
> > > > > When going to sleep the sgx_pm_cnt is increased. sgx_einit()
> > > > > compares the current value of the global count to the value in
> > > > > the cache entry to see whether we are in a new power cycle.
> > > >
> > > > You mean reset to Intel default? I think we can also just reset
> > > > the cached MSR values on each power cycle, which would be simpler,
> IMHO?
> > >
> > > Refresh my brain, does hardware reset the MSRs on a transition to S3 or
> lower?

Sorry I missed this one. To be honest I don't know. I checked the SDM and all I can find is:

"On reset, the default value is the digest of Intel's signing key."

Jarkko may know.

> > >
> > > > I think we definitely need some code to handle S3-S5, but should
> > > > be in separate patches, since I think the major impact of S3-S5 is
> > > > entire EPC being destroyed. I think keeping pm_cnt is not
> > > > sufficient enough to handle such case?
> > > > >
> > > > > This brings up one question though: how do we deal with VM host
> > > > > going to
> > > sleep?
> > > > > VM guest would not be aware of this.
> > > >
> > > > IMO VM just gets "sudden loss of EPC" after suspend & resume in host.
> > > > SGX driver and SDK should be able to handle "sudden loss of EPC",
> > > > ie, co-working together to re-establish the missing enclaves.
> > > >
> > > > Actually supporting "sudden loss of EPC" is a requirement to
> > > > support live migration of VM w/ SGX. Internally long time ago we
> > > > had a discussion and the decision was we should support SGX live
> > > > migration given
> > > two facts:
> > > >
> > > > 1) losing platform-dependent is not important. For example, losing
> > > > sealing key is not a problem, as we could get secrets provisioned
> > > > again from remote. 2) Both windows & linux driver commit to
> > > > support "sudden
> > > loss of EPC".
> > > >
> > > > I don't think we have to support in very first upstream driver,
> > > > but I think we need to support someday.
> > >
> > > Actually, we can easily support this in the driver, at least for SGX1 hardware.
> >
> > That's my guess too. Just want to check whether we are still on the
> > same page :)
> >
> > > SGX2 isn't difficult to handle, but we've intentionally postponed
> > > those patches until SGX1 support is in mainline[1].
> > > Accesses to the EPC after it is lost will cause faults.  Userspace EPC accesses,
> e.g.
> > > ERESUME, will get a SIGSEGV that the process should interpret as an
> > > "I should restart my enclave" event.  The SDK already does this.  In
> > > the driver, we just need to be aware of this potential behavior and
> > > not freak out.  Specifically, SGX_INVD needs to not WARN on faults that may
> have been due to a the EPC being nuked.
> > > I think we can even remove the sgx_encl_pm_notifier() code altogether.
> >
> > Possibly we still need to do some cleanup, ie, all structures of enclaves, upon
> resume?
> 
> Not for functional reasons.  The driver will automatically do the cleanup via
> SGX_INVD when it next accesses the enclave's pages and takes a fault, e.g.
> during reclaim.  Proactively reclaiming the EPC pages would probably affect
> performance, though not necessarily in a good way.  And I think it would be a
> beneficial to get the driver out of the suspend/hibernate/resume paths, e.g.
> zapping all enclaves could noticeably impact suspend/resume latency.

Sure.

Thanks,
-Kai

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