[PATCH RFC 1/3] mm: Add f_ops->populate()

[Jarkko Sakkinen]

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/#m195ec84bf85614a140abeee245c5118c22ace8f3

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