GCC Compiler Adds Software Workaround To Avoid Intel Downfall Performance Hit

[phoronix]

Phoronix: GCC Compiler Adds Software Workaround To Avoid Intel Downfall Performance Hit

With the Intel Downfall vulnerability made public last week (also known as GDS - "Gather Data Sampling") there can be a sizable hit to AVX workloads making use of the GATHER instructions. For helping to lower the impact of Downfall/GDS on mitigated systems, Intel has made a change to the GNU Compiler Collection to disable GATHER generation in vectorization for Intel CPU families affected by this vulnerability...

https://www.phoronix.com/news/GCC-Workaround-Intel-Downfall

[OpenSourceAnarchist]

Considering many non-commercial Linux users here likely use "mitigations=off" anyway, I'm a little disappointed in this despite it likely being for the best. I'm not even "allowed" by Intel to make use of the hardware abilities, unless I recompile everything. My ideal distro would be a source-based Clear Linux hybrid to avoid having to use precompiled binaries with default patches/build options. Something like SerpentOS or Solus with a ports system, and a GentooLTO mentality

[baka0815]

Michael

is used in palce of

"is used in place of"

[bezirg]

Considering many non-commercial Linux users here likely use "mitigations=off" anyway, I'm a little disappointed in this despite it likely being for the best. I'm not even "allowed" by Intel to make use of the hardware abilities, unless I recompile everything. My ideal distro would be a source-based Clear Linux hybrid to avoid having to use precompiled binaries with default patches/build options. Something like SerpentOS or Solus with a ports system, and a GentooLTO mentality

A flag should be made (if it is not already there) that acts similar to kernel's `mitigations=off`, but for GCC.

[coder]

Considering many non-commercial Linux users here likely use "mitigations=off" anyway, I'm a little disappointed in this despite it likely being for the best.

You should still be able to avoid the vast majority of performance impact, by simply by disabling the mitigation. I think Intel said their downfall mitigation is runtime-selectable, though I'm not sure if they've yet followed-through on that, what the details are for disabling it, or if it's included in mitigations=off.

I'm not even "allowed" by Intel to make use of the hardware abilities, unless I recompile everything.

Nope. That's not what the patch does. All it does is prevent the compiler's auto-vectorization from generating GATHER instructions. However, most of the optimized, vectorized code uses hand-coded intrinsics or assembly language, which aren't affected by this. Auotvectorization typically doesn't work terribly well, which is why people tend not to depend on it, when it really matters.

[HadrienG]

I'm amazed people managed to get the GCC autovectorizer to generate GATHER instructions at all. In my experience, it has always given up as soon as irregular memory accesses or branching were involved.