Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

From: H. Peter Anvin
Date: Tue May 20 2008 - 11:00:45 EST

Andi Kleen wrote:
Ok. CPU folks are planning to make some of the bytes at the end of fxsave
image, SW usable.

Are they always zeroed in earlier CPUs though? If not that wouldn't
work 100% reliably because whatever cookie you put in could have been
there before by chance.

I don't see anything in the SDM guaranteeing zeroing.

I'm pretty sure they weren't zeroed by the CPUs. If they weren't zeroed *by the kernel*, there might have been an information leak.

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