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

["Haitao Huang" <haitao.huang@linux.intel.com>]

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);

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.

Thanks
Haitao