linux-kernel - Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

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Date:	Mon, 21 Dec 2015 15:00:08 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

On 12/21/2015 02:52 PM, Andy Lutomirski wrote:
> Perhaps this is silly, but what if the default were changed to deny
> reads and writes for unallocated keys?  Is there a use case that
> breaks?

It's probably a reasonable debugging feature.

But, anything that takes an XSAVE feature out of its "init state" has
the potential to do a bit of harm because it increases the potential
size of writes during XSAVE.  XSAVEOPT will _help_ here, but we probably
don't want to go out of our way to take things out of the init state
when we're unsure of the benefits.
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