On 4/2/21 11:31 AM, Jethro Beekman wrote: > On 2021-04-02 17:53, Dave Hansen wrote: >> On 4/2/21 1:38 AM, Jethro Beekman wrote: >>>> So, we're talking here about pages that have been EEADDED, but for >>>> which we do not want to include the entire contents of the page? >>>> Do these contents always include the beginning of the page, or can >>>> the holes be anywhere? >>> Holes can be anywhere, and EEXTEND calls need not be sequential in >>> memory address or even relate to the most recently EADDed page. >> >> I think you're referring to the SGX architecture itself here. The >> architecture permits this, right? > > Yes. > >> But, why would an enclave loader application ever do this? > > e.g. to save space
How does this save space, exactly? What space does it save? Let's say I want to add 4352 bytes of data to an enclave. Today, I have to page-align the beginning and end of that 4352 bytes, and call the add ioctl() to add the two resulting pages. It consumes two EPC pages. With EEXTEND, if I want to add the data, I only need to page-align the beginning of the data. I call add_page on the first page, then eextend on the 256 bytes. It consumes two EPC pages. I guess you can argue that this saves padding out the second page, which would *theoretically* temporarily eat up one extra page of non-enclave memory and the cost of a 256-byte memcpy. >> Is this something we want to support in Linux? > > Why not? Is there a good reason to not fully support this part of the > CPU architecture? We don't blindly support CPU features in Linux. They need to do something *useful*. I'm still missing what this does which is useful. Does any actual, real-world enclave want this functionality? Why? P.S. There are plenty of things you can do with the SGX architecture that we probably won't ever implement in Linux.
