Re: [PATCH RFC] x86: Add SGX_IOC_ENCLAVE_AUGMENT_PAGES

[Haitao Huang]

On Fri, 04 Mar 2022 19:26:12 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx>  
wrote:

> On Fri, Mar 04, 2022 at 10:27:58AM -0600, Haitao Huang wrote:
> > On Fri, 04 Mar 2022 06:28:52 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > 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  
> > with
> > > SGX_IOC_ENCLAVE_AUGMENT_PAGES, which performs EAUG's to a given range  
> > of
> > > pages. Do not enforce any particular permissions from kernel, like is
> > > done
> > > for the pages added during the pre-initialization phase, as enclave
> > > controls the final permissions and content for these pages by issuing
> > > either ENCLU[EACCEPT] (empty RW) or ENCLU[EACCEPTCOPY] (arbitrary data
> > > and
> > > permissions).
> > >
> > > Explicit EAUG ioctl is a better choice than an implicit EAUG from a  
> > page
> > > fault handler because it allows to have O(1) number of kernel-enclave
> > > round
> > > trips for EAUG-EACCEPT{COPY} process, instead of O(n), as it is in the
> > > case
> > > when a page fault handler EAUG single page at a time.
> > >
> > > Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> > > Cc: Nathaniel McCallum <nathaniel@xxxxxxxxxxx>
> > > Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > > ---
> > > Is contained in sgx2-v2.1 branch of
> > > git://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-sgx.git
> > > ---
> > >  arch/x86/include/uapi/asm/sgx.h |  14 +++
> > >  arch/x86/kernel/cpu/sgx/ioctl.c | 159  
> > ++++++++++++++++++++++++++++++++
> > >  2 files changed, 173 insertions(+)
> > >
> > > diff --git a/arch/x86/include/uapi/asm/sgx.h
> > > b/arch/x86/include/uapi/asm/sgx.h
> > > index c4e0326d281d..2b3a606e78fe 100644
> > > --- a/arch/x86/include/uapi/asm/sgx.h
> > > +++ b/arch/x86/include/uapi/asm/sgx.h
> > > @@ -35,6 +35,8 @@ enum sgx_page_flags {
> > >  	_IOWR(SGX_MAGIC, 0x06, struct sgx_enclave_modt)
> > >  #define SGX_IOC_ENCLAVE_REMOVE_PAGES \
> > >  	_IOWR(SGX_MAGIC, 0x08, struct sgx_enclave_remove_pages)
> > > +#define SGX_IOC_ENCLAVE_AUGMENT_PAGES \
> > > +	_IOWR(SGX_MAGIC, 0x09, struct sgx_enclave_augment_pages)
> > > /**
> > >   * struct sgx_enclave_create - parameter structure for the
> > > @@ -138,6 +140,18 @@ struct sgx_enclave_remove_pages {
> > >  	__u64 count;
> > >  };
> > > +/**
> > > + * struct sgx_enclave_augment_pages - parameter structure for the
> > > %SGX_IOC_ENCLAVE_ADD_PAGE ioctl
> > > + * @offset:	starting page offset
> > > + * @length:	length of the data (multiple of the page size)
> > > + * @count:	number of bytes added (multiple of the page size)
> > > + */
> > > +struct sgx_enclave_augment_pages {
> > > +	__u64 offset;
> > > +	__u64 length;
> > > +	__u64 count;
> > > +};
> > > +
> >
> > As I stated in another thread, we need a mechanism to allow EAUG page
> > lazily, e.g., on #PF. Can we add a field here to indicate that?
>
> ioctl *does not* prevent lazy behaviour where, or if, it makes sense.
>
> For growing memory (e.g. MAP_GROWSDOWN) you should just take advantage of
> the vDSO's exception handling mechanism and call the ioctl on demand.
>
> For a high-performance user space you still want to be also do minimum
> round trip "batch jobs" where they are possible.

Looks like you are pursuing MAP_POPULATE to optimize out the O(N) trips.
Just for my understanding of your proposal in case this ever comes back.  
For the on-demand case,  this ioctl is required for for each #PF. That's  
extra round trip compared to automatic kernel EAUG on #PF.


> We exactly have the whole vDSO framework to service the on-demand needs
> while still having full control of the execution. EAUG in the #PF handler
> is all about being flakky and loosing all robustness.

Again, you keep saying EAUG in the #PF handler is not good. So far the  
only concrete thing I hear is extra O(N) round trips which can be  
optimized out.