Re: [PATCH v13 09/13] x86/sgx: Enclave Page Cache (EPC) memory manager

[Jarkko Sakkinen]

On Tue, Aug 28, 2018 at 02:22:44PM -0700, Sean Christopherson wrote:
> On Tue, Aug 28, 2018 at 07:07:33AM -0700, Dave Hansen wrote:
> > On 08/28/2018 01:35 AM, Jarkko Sakkinen wrote:
> > > On Mon, Aug 27, 2018 at 02:15:34PM -0700, Dave Hansen wrote:
> > >> On 08/27/2018 11:53 AM, Jarkko Sakkinen wrote:
> > >>> +struct sgx_epc_page_ops {
> > >>> +	bool (*get)(struct sgx_epc_page *epc_page);
> > >>> +	void (*put)(struct sgx_epc_page *epc_page);
> > >>> +	bool (*reclaim)(struct sgx_epc_page *epc_page);
> > >>> +	void (*block)(struct sgx_epc_page *epc_page);
> > >>> +	void (*write)(struct sgx_epc_page *epc_page);
> > >>> +};
> > >> Why do we need a fancy, slow (retpoline'd) set of function pointers when
> > >> we only have one user of these (the SGX driver)?
> > > KVM has its own implementation for these operations.
> > 
> > That belongs in the changelog.
> > 
> > Also, where is the implementation?  How can we assess this code that was
> > built to create an abstraction without both of the users?
> 
> I can provide an early preview of the KVM reclaim code, but honestly
> I think that would do more harm than good.  The VMX architecture for
> EPC reclaim is complex, even for SGX standards.  Opening that can of
> worms would likely derail this discussion.  That being said, this
> abstraction isn't exactly what KVM will need, but it's pretty close
> and gives us something to build on.
> 
> Regardless, this layer of indirection is justifiable even with a
> single implementation.  Reclaiming an EPC page is not a simple matter
> of copying the data somewhere else and marking the page not present.
> Actual eviction requires a reference to the Secure Enclave Control
> Structure (SECS) of the enclave that owns the page.  Software needs
> to ensure it doesn't violate the hardware-enforced access rules, e.g.
> most reclaim activites need to be done under a per-enclave lock.
> Not all pages have the same reclaim rules, e.g. an SECS can only
> be reclaimed after all its child pages have reclaimed, a VA page
> doesn't need to be blocked, etc...  And the list goes on...

To simplify a bit what Sean said about the key difference to a standard
page is that in SGX a page is part of a hierarchical structure. EPC
pages have hardware enforced dependencies to each other.

Examples: 

1. You cannot delete or swap SECS before its children have been deleted or
   swapped. You get SGX_CHILD_PRESENT erro from EREMOVE.
2. In order to swap or fault a page you need to have an EPC page that
   holds a version number for the page you want to swap. These are
   called Version Array (VA) pages.

/Jarkko