Re: [PATCH 13/25] x86/sgx: Support adding of pages to initialized enclave

[Jarkko Sakkinen]

"to initialize" -> "to an initialized"

On Wed, Dec 01, 2021 at 11:23:11AM -0800, Reinette Chatre wrote:
> With SGX1 an enclave needs to be created with its maximum memory demands
> allocated. Pages cannot be added to an enclave after it is initialized.
> SGX2 introduces a new function, ENCLS[EAUG], that can be used to add
> pages to an initialized enclave. With SGX2 the enclave still needs to
> set aside address space for its maximum memory demands during enclave
> creation, but all pages need not be added before enclave initialization.
> Pages can be added during enclave runtime.
> 
> Add support for dynamically adding pages to an initialized enclave,
> architecturally limited to RW permission. Add pages via the page fault
> handler at the time an enclave address without a backing enclave page
> is accessed, potentially directly reclaiming pages if no free pages
> are available.
> 
> The enclave is still required to run ENCLU[EACCEPT] on the page before
> it can be used. A useful flow is for the enclave to run ENCLU[EACCEPT]
> on an uninitialized address. This will trigger the page fault handler
> that will add the enclave page and return execution to the enclave to
> repeat the ENCLU[EACCEPT] instruction, this time successful.
> 
> If the enclave accesses an uninitialized address in another way, for
> example by expanding the enclave stack to a page that has not yet been
> added, then the page fault handler would add the page on the first
> write but upon returning to the enclave the instruction that triggered
> the page fault would be repeated and since ENCLU[EACCEPT] was not run
> yet it would trigger a second page fault, this time with the SGX flag
> set in the page fault error code. This can only be recovered by entering
> the enclave again and directly running the ENCLU[EACCEPT] instruction on
> the now initialized address.
> 
> Accessing an uninitialized address from outside the enclave also triggers
> this flow but the page will remain in PENDING state until accepted from
> within the enclave.

What does it mean being in PENDING state, and more imporantly, what is
PENDING state? What does a memory access within enclave cause when it
touch a page within this state?

I see a lot of text in the commit message but zero mentions about EPCM
expect this one sudden mention about PENDING field without attaching
it to anything concrete.

/Jarkko