Messages in this thread | | | Subject | Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues | From | Dave Hansen <> | Date | Fri, 29 Apr 2016 11:09:23 -0700 |
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Hi Folks,
I've heard through the grapevine that there's some concern that we should not be bothering to enable XSAVES because there's not a sufficient use case for it. Maybe it's meager today, but I still think we should do it.
I'll try to lay out why.
Today, on every Skylake system, this patch saves 128 bytes in each task_struct. If there were an Atom system with XSAVES it would save 384 bytes since there is no AVX support on Atom. If there were a future processor which has an xstate _past_ AVX-512, but that does not have AVX-512 itself, that savings goes up to 2048+384 bytes. I believe it is *inevitable* that the savings will become substantial.
Plus, if the processors ever start supporting a supervisor state that we _need_ in Linux, we have to XSAVES support anyway.
It's inevitable that we _will_ need it.
Why do it today?
Now that Skylake is out, we _can_ get reasonable testing of this feature from early adopters in the wild. If we turn this on today, and it breaks, we break a relatively modest number of Skylake systems (1%? 2%? 0.1%?). Let's say we wait $X years when the benefits are greater. We turn it on, and something breaks. We'll break 50% (or 40% or whatever) of the systems in production.
Once we *HAVE* XSAVES support, it also opens up the possibilities for doing things like dynamic XSAVE buffer allocation. For instance, let threads that are not _using_ AVX-512 not waste the 2k of space for it.
So why wait?
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