Re: regression in TCG emulation of VTBL neon instruction

["Alex Bennée" <alex.bennee@linaro.org>]

Ard Biesheuvel <ardb@kernel.org> writes:

> Hello all,
>
> I spotted an issue with the TCG emulation of VTBL instructions in 32-bit mode.
>
> It seems that when using the 4 register version, indexes in the range
> [0x10 .. 0x1f] are not handled correctly, and I end up with all zero
> vectors in the output.
>
> For example, I am optimizing Linux's NEON ChaCha20 implementation to
> use overlapping loads and stores, and this requires the final cipher
> stream block to be shifted accordingly, using a sequence such as
>
> vtbl.8 d4, {q4-q5}, d4
> vtbl.8 d5, {q4-q5}, d5
> vtbl.8 d6, {q4-q5}, d6
> vtbl.8 d7, {q4-q5}, d7
>
> where q4-q5 contain 32 bytes of cipher stream, and d4-d7 contain a set
> of permutation vectors, where each value is in the range [0x0, 0x1f].
>
> The above works fine with older QEMU and KVM, but with recent QEMU,
> this fails, seemingly because d6 and d7 always turn up as all zeros.
>
> This can be reproduced by running the zImage I prepared [0] as follows:
>
> qemu-system-aarch64 -M virt -cpu cortex-a15 -m 2048 -net none
> -nographic -kernel arch/arm/boot/zImage
>
> and it will print the following (somewhere halfway down the kernel
> log) on the affected builds of QEMU:
>
> alg: skcipher: chacha20-neon encryption test failed (wrong result) on
> test vector 1, cfg="in-place"
> alg: skcipher: xchacha20-neon encryption test failed (wrong result) on
> test vector 1, cfg="in-place"
> alg: skcipher: xchacha12-neon encryption test failed (wrong result) on
> test vector 1, cfg="in-place"

I get:

[    8.974879] testing speed of sync chacha20 (chacha20-neon) encryption
[    8.975230] tcrypt: test 0 (256 bit key, 16 byte blocks): 351309 operations in 1 seconds (5620944 bytes)
[    9.967242] tcrypt: test 1 (256 bit key, 64 byte blocks): 383886 operations in 1 seconds (24568704 bytes)
[   10.967103] tcrypt: test 2 (256 bit key, 256 byte blocks): 109213 operations in 1 seconds (27958528 bytes)
[   11.967164] tcrypt: test 3 (256 bit key, 1024 byte blocks): 29061 operations in 1 seconds (29758464 bytes)
[   12.967165] tcrypt: test 4 (256 bit key, 1420 byte blocks): 19577 operations in 1 seconds (27799340 bytes)
[   13.967147] tcrypt: test 5 (256 bit key, 4096 byte blocks): 7217 operations in 1 seconds (29560832 bytes)
[   14.972354] input: gpio-keys as /devices/platform/gpio-keys/input/input0
[   14.977272] uart-pl011 9000000.pl011: no DMA platform data
[   14.980208] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[   14.980431] Please append a correct "root=" boot option; here are the available partitions:

I wonder if it was a transient bug when stuff was converted to
decodetree and got fixed up later? Tested on HEAD @ 4c5b97bfd and @
e46912b66.

>
>
>
> [0] https://people.linaro.org/~ard.biesheuvel/qemu-tcg-vtbl/zImage


-- 
Alex Bennée