Re: [RFC PATCH v4 2/4] x86/sgx: Implement support for MADV_WILLNEED

Huang, Kai
2023-02-14T22:37:46Z

On Tue, 2023-02-14 at 15:42 -0600, Haitao Huang wrote:
> On Tue, 14 Feb 2023 14:54:53 -0600, Huang, Kai <kai.huang@intel.com> wrote:
>
> > On Tue, 2023-02-14 at 13:18 -0600, Haitao Huang wrote:
> > > Hi Kai
> > >
> > > On Tue, 14 Feb 2023 03:47:24 -0600, Huang, Kai <kai.huang@intel.com>
> > > wrote:
> > >
> > > > On Fri, 2023-01-27 at 20:55 -0800, Haitao Huang wrote:
> > > > > @@ -97,10 +99,81 @@ static int sgx_mmap(struct file *file, struct
> > > > > vm_area_struct *vma)
> > > > > vma->vm_ops = &sgx_vm_ops;
> > > > > vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP | VM_IO;
> > > > > vma->vm_private_data = encl;
> > > > > + vma->vm_pgoff = PFN_DOWN(vma->vm_start - encl->base);
> > > > > return 0;
> > > > > }
> > > >
> > > > Perhaps I am missing something, but above change looks weird.
> > > > Conceptually, it doesn't/shouldn't belong to this series, which
> > > > essentially
> > > > preallocates and does EAUG EPC pages for a (or part of) given enclave.
> > > > The EAUG
> > > > logic should already be working for the normal fault path, which means
> > > > the code
> > > > change above either: 1) has been done at other place; 2) isn't needed.
> > > >
> > > > I have kinda forgotten the userspace sequence to create an enclave.
> > > If
> > > > I recall
> > > > correctly, you do below to create an enclave:
> > > >
> > > > 1) encl_fd = open("/dev/sgx_enclave");
> > > > 2) encl_addr = mmap(encl_fd, encl_size, 0 /* pgoff */);
> > > > 3) IOCTL(ECREATE, encl_addr, encl_size);
> > > >
> > > > Would the above code change break the "mmap()" in above step 2?
> > > >
> > >
> > > No, vm_pgoff was not used previously for enclave VMAs. I had to add this
> > > because the offset passed to sgx_fadvise is relative to file base and
> > > calculated in mm/madvise.c like this:
> > >
> > > offset = (loff_t)(start - vma->vm_start)
> > > + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> >
> > But shouldn't 'offset is relative to the file base' be conceptually
> > correct from
> > the fadvice()'s point of view?
> >
> > I think you should do:
> >
> > encl_offset = offset + encl->base;
> >
> > inside sgx_fadvice()?
> >
> > >
> If we don't set vma->vm_pgoff (default to zero), then offset will be
> calculated as (start - vma->vm_start). Then the above calculation is wrong
> if we have multiple VMAs for the same enclave, which is usually the case.

do_mmap() -> mmap_region() itself sets vma->vm_pgoff:

vma = vm_area_alloc();
...
vma->vm_pgoff = pgoff;

if (file)
call_mmap(file, vma); <- sgx_mmap()

I think you will always call mmap() against enclave's fd with 'pgoff' being set
to the offset relative to the file?

>
> > > I had a comment in first version but removed it based on Jarkko's
> > > suggestion here:
> > > https://lore.kernel.org/all/Y2B0jBsG6HE4KVk7@kernel.org/
> > >
> > > The original comments probably seemed redundant to the definitions of
> > > the
> > > vm_pgoff field and the fadvise interface. But let me know if we need
> > > add a
> > > more helpful version of comments or any suggestion on the comments.
> >
> > I still think this code change is wrong.
> >
> > For instance, IIUC, it at least breaks the case where enclave hasn't been
> > created/initialized, where encl->base == 0 (although normal code path
> > doesn't
> > use vm_pgoff, conceptually it's still wrong IIUC).
> >
> > Maybe I am missing something?
>
> The fadvise interface is only usable for an initialized enclave,
> sgx_fadvise will return error otherwise.
>

True. But that code change is unconditionally called for all mmap(), even when
enclave hasn't been created.

> Conceptually I view enclave base
> as "file base", it's just that we don't ever need handle the zero case
> caused by uninitialized enclave (kind of like a file never mapped). If an
> initialized enclave happens to have zero base, it would also work.

A little bit confused about what does "enclave base" here.

To me, A file is an enclave, meaning the "file offset" equals to "enclave
offset". "enclave base" is the base linear address of the enclave, it doesn't
matter whether it is 0 or not. You get an "enclave address" from "enclave base"
plus "enclave offset" (or "file offset"):

enclave_addr = enclave_base + enclave_offset/file_offset;

And such calculation is only valid after enclave has been created (enclave_base
is valid -- can be 0 or whatever).

Since sgx_mmap() can happen before enclave is created, calculating the vm_pgoff
from enclave_base is conceptually wrong. Even if you really want to do it, it
should be:

if (enclave_has_initialized())
vma->vm_pgoff = ...;

But again I am not convinced why you cannot get the enclave_addr inside
sgx_fadvice().