Improve protected files performance

[yao-ji]

Description of the feature

Current implementation of protected file is slow, especially for large files. At least the following 2 improvements can be applied.

1. Use file map to replace read/write a single node from/to disk.
2. Remove the encrypted part inside the file_node_t structure. The encrypted content can be directly retrieved from disk. The encryption/decryption functions can use the file mapped addresses instead so we can save the cache memory as well as the time used to copy it to/from disk.

Why Gramine should implement it?

Performance test on SGX SDK shows that applying these two changes can make reading 2 times fasters and writing 7 times faster.

[dimakuv]

Looks like reasonable optimizations.

[dimakuv]

@yao-ji Is this something that you want to work on?

[yao-ji]

@yao-ji Is this something that you want to work on?

Yes. And I have collected some data about time can be saved after applying the changes.

Since there is no need to call g_cb_read and g_cb_write in reading/writing each node, we calculate how much time these two call backs used inside each read/write syscall. Note that the write syscall performs both ipf_write and ipf_internal_flush, so we will add the time used of these two together. The test code can be found here.

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File size	8KB	32KB	128KB	512KB	2MB	8MB	32MB
write time used (ns)	3453	8765	29800	118334	497853	1942698	8032618
write time saved (ns)	536	1307	4208	17014	70953	279094	1109268
saved percentage	15.5227%	14.9116%	14.1208%	14.3779%	14.2518%	14.3663%	13.8095%
read time used (ns)	1627	5958	25457	95566	417476	1591918	6287079
read time saved (ns)	160	257	681	2408	9459	36452	151601
saved percentage	9.8341%	4.3135%	2.6751%	2.5197%	2.2658%	2.2898%	2.4113%