Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues

[Dave Hansen]

On 04/29/2016 12:57 PM, Yu-cheng Yu wrote:
> On Fri, Apr 29, 2016 at 11:09:23AM -0700, Dave Hansen wrote:
>> 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.
> 
> If we can somehow modify exec* system call to scan the executable
> binary (in user space) and pass along a bitmask containing xfeatures
> used in the binary, and XSAVES is enabled in the kernel, we can
> easily save a lot of memory.  The kernel only needs to allocate space
> for tasks that actually use xstates; most of them do not.

That's not feasible.  Think of dynamic libraries or just-in-time
compilers.  What instruction set does /usr/bin/java use, for instance? :)