linux-kernel - Re: [PATCH v13 10/13] x86/sgx: Add sgx

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Date:   Fri, 31 Aug 2018 10:43:30 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     "Huang, Kai" <kai.huang@...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

On Wed, Aug 29, 2018 at 06:45:29PM -0700, Huang, Kai wrote:
> > > > > > 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."

I confirmed the MSRs are reset any time the EPC is lost.  Not sure
what happens if the MSRs contained a non-Intel value but feature
control is locked with SGX launch control disabled.  I'll post an
update when I have an answer.
 
> 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
>