On Sun, Mar 06, 2022 at 10:43:31PM +0000, Matthew Wilcox wrote:
On Sun, Mar 06, 2022 at 07:02:57PM +0200, Jarkko Sakkinen wrote:
So can I conclude from this that in general having populate available for device memory is something horrid, or just the implementation path?
You haven't even attempted to explain what the problem is you're trying to solve. You've shown up with some terrible code and said "Hey, is this a good idea". No, no, it's not.
The problem is that in order to include memory to enclave, which is essentially a reserved address range processes virtual address space there's two steps into it:
1. Host side (kernel) does ENCLS[EAUG] to request a new page to be added to the enclave. 2. Enclave accepts request with ENCLU[EACCEPT] or ENCLU[EACCEPTCOPY].
In the current SGX2 patch set this taken care by the page fault handler. I.e. the enclave calls ENCLU[EACCEPT] for an empty address and the #PF handler then does EAUG for a single page.
So if you want to process a batch of pages this generates O(n) round-trips.
So if there was a way pre-do a batch of EAUG's, that would allow to load data to the enclave without causing page faults happening constantly.
One solution for this simply add ioctl:
https://lore.kernel.org/linux-sgx/YiLRBglTEbu8cHP9@iki.fi/T/#m195ec84bf85614...
But in practice when you wanted to use it, you would setup the parameters so that they match the mmap() range. So for pratical user space API having mmap() take care of this would be much more lean option.
BR, Jarkko