Re: [PATCH v4 1/4] Documentation/x86: Explain the purpose for dynamic features

From: Dave Hansen
Date: Fri Sep 09 2022 - 17:36:13 EST

On 9/9/22 13:15, Chang S. Bae wrote:
> +The purpose for dynamic features
> +--------------------------------
> +
> + - Legacy userspace libraries have hard-coded sizes for an alternate signal
> + stack. With the arch_prctl() options, the signal frame beyond AVX-512
> + and PKRU will not be written by old programs as they are prevented from
> + using dynamic features. Then, the small signal stack will be compatible
> + on systems that support dynamic features.

This doesn't really ever broach the _problem_ that dynamic features solve.

Legacy userspace libraries often have hard-coded, static sizes
for alternate signal stacks, often using MINSIGSTKSZ which is
typically 2k. That stack must be able to store at *least*
the signal frame that the kernel sets up before jumping into
the signal handler. That signal frame must include an XSAVE
buffer defined by the CPU.

However, that means that the size of signal stacks is dynamic,
not static, because different CPUs have differently-sized XSAVE
buffers. Those old <=2k buffers are now too small for new CPU
features like AVX-512, which is causing stack overflows at
signal entry.

> + - Modern server systems are consolidating more applications to share the
> + CPU resource.

I'm not sure what this means. Are you saying that CPU time is more
overcommitted? Or that different users are more likely to be sharing
the same CPU core? Or, is this trying to allude to the frequency
penalties that cores (and even packages) pay for using features like

> The risk of applications interfering with each other is
> + growing. The controllability on the resource trends to be more
> + warranted. Thus, this permission mechanism will be useful for that.

Should this be something more like:

Historically, a CPU shared very few resources with its neighbors outside
of caches. A CPU could execute whatever instructions it wanted without
impacting other CPUs. Also, there were minimal long-lasting temporal
effects; an application that preceded yours running on a CPU would not
impact how your application runs.

That model has been eroding, first with SMT where multiple logical CPUs
share a core's resources. Then, with features like AVX-512 that have a
frequency and thermal impact which can last even after AVX-512 use
ceases and have an impact wider than a single core.

In other words, it has become easier to be a "noisy neighbor".

Dynamic features allow the kernel limit applications' ability to become
noisy neighbors in the first place.