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Clarify AES GFNI performance expectations and document RAID affine multiplication behavior

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Andrea Mazzoleni 2026-08-23 20:26:14 +02:00
parent a745a05d4e
commit 63c81dedf3
3 changed files with 34 additions and 19 deletions
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20
raid/README
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@ -122,15 +122,21 @@ Operating Modes and Polynomials
RAID_MODE_CAUCHY_AES (Mode = 2)
Uses Cauchy matrix with AES polynomial 0x1b (x^8 + x^4 + x^3 + x + 1,
0x11b) and primitive generator g = 3. On CPUs supporting Intel GFNI,
generation and recovery are accelerated directly using VGF2P8MULB.
This native GFNI multiplication is typically faster than the affine GFNI
implementation required by the standard RAID polynomial. Parity data is
generation and recovery can use VGF2P8MULB directly for multiplication
in the AES field. This mode also provides a useful benchmark and
reference implementation for comparison with RAID_MODE_CAUCHY_RAID,
where fixed-coefficient multiplication in the 0x11d field is implemented
with VGF2P8AFFINEQB and precomputed affine matrices. Relative performance
depends on the CPU, backend, parity count, and workload. Parity data is
incompatible with standard 0x11d polynomial arrays.
Both Cauchy modes use the same Extended Cauchy construction with coefficients
based on powers of their respective primitive generator. GFNI acceleration
is available for both fields: the RAID field uses affine transformations,
while the AES field uses the native GFNI multiplication instruction.
Both Cauchy modes use the same Extended Cauchy construction with
coefficients based on powers of their respective primitive generators.
GFNI acceleration is available for both fields: the RAID field uses
VGF2P8AFFINEQB for fixed-coefficient multiplication, while the AES field
can use native VGF2P8MULB. Neither instruction should be assumed to be
faster in general; relative performance is hardware and workload
dependent.
Implementation Structure

2
raid/mktables.c
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@ -8,7 +8,7 @@
* Galois field reduction polynomials.
*
* RAID_POLY_RAID - Standard RAID polynomial 0x1d (x^8 + x^4 + x^3 + x^2 + 1).
* RAID_POLY_AES - AES polynomial 0x1b (x^8 + x^4 + x^3 + x + 1) for GFNI acceleration.
* RAID_POLY_AES - AES polynomial 0x1b (x^8 + x^4 + x^3 + x + 1), using generator g=3.
*/
#define RAID_POLY_RAID 0x1d
#define RAID_POLY_AES 0x1b

31
raid/raid.h
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@ -35,18 +35,22 @@
*
* On CPUs supporting Intel GFNI, generation and recovery can use the
* vgf2p8mulb instruction, which performs GF(2^8) multiplication directly
* in hardware using this polynomial. This native multiplication is
* typically faster than the affine GFNI implementation required for the
* standard RAID polynomial.
* using this polynomial.
*
* This mode also provides a useful comparison with
* RAID_MODE_CAUCHY_RAID, where multiplication by a fixed coefficient in
* GF(2^8)/0x11d is implemented directly with vgf2p8affineqb and a
* precomputed 8x8 GF(2) matrix. Relative performance depends on the CPU,
* backend, parity count, and workload.
*
* The trade-off is that parity data is not compatible with arrays using
* the standard 0x11d polynomial (g=2), so all data must be generated and
* recovered using the same operating mode.
*
* Both Cauchy modes can therefore use GFNI acceleration. The standard RAID
* mode preserves compatibility by using affine GFNI transformations, while
* the AES mode trades compatibility for the typically faster native GFNI
* multiplication instruction.
* mode uses affine transformations for fixed-coefficient multiplication in
* the 0x11d field, while the AES mode can use native vgf2p8mulb
* multiplication in the 0x11b field.
*/
/**
@ -76,12 +80,17 @@
#define RAID_MODE_VANDERMONDE_RAID 1
/**
* RAID mode supporting up to 6 parities using AES polynomial 0x1b (0x11b) and primitive generator g=3.
* RAID mode supporting up to 6 parities using AES polynomial 0x1b (0x11b)
* and primitive generator g=3.
*
* It has slightly lower performance than RAID_MODE_CAUCHY_RAID on CPUs without GFNI due
* to the additional cost of generator g=3. On CPUs supporting GFNI, it uses native
* VGF2P8MULB multiplication and is typically faster than the affine GFNI implementation
* required by RAID_MODE_CAUCHY_RAID.
* It has slightly lower performance than RAID_MODE_CAUCHY_RAID on CPUs
* without GFNI due to the additional cost of generator g=3.
*
* On CPUs supporting GFNI, it uses native VGF2P8MULB multiplication. This
* mode also provides a useful benchmark and reference implementation for
* comparing native AES-field GFNI multiplication with the affine GFNI
* implementation used by RAID_MODE_CAUCHY_RAID. Relative performance
* depends on the CPU, backend, parity count, and workload.
*
* Parity data is incompatible with RAID_MODE_CAUCHY_RAID.
*/