linux-kernel - Re: [RFC] Restore PKRU to user-defined value after signal handling

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Message-ID: <59464733-17d1-4b43-83f4-e85a8389638d@intel.com>
Date: Thu, 7 Nov 2024 16:26:15 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
Cc: Rudi Horn <rudi.horn@...cle.com>, Thomas Gleixner <tglx@...utronix.de>,
 "mingo@...hat.com" <mingo@...hat.com>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
 "x86@...nel.org" <x86@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Joe Jin <joe.jin@...cle.com>, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [RFC] Restore PKRU to user-defined value after signal handling

On 11/7/24 15:56, Aruna Ramakrishna wrote:
> If it is calcuated by the kernel, is there a chance that we could inadvertently
> set XINUSE[i] to 1 for more components other than just PKRU? Since it is 
> possible that some other component was set to its init state by XSAVE, 

XINUSE is exposed in the ISA via XGETBV(1). If it were _totally_ racy
and the CPU could change it willy nilly at any time, it couldn't be
sanely exposed.

I think it's safe to assume that if you use XGETBV(1) that the state is
sticky at least until there's an explicit change to a state component.

If you're really worried about this, we could go ask the hardware folks.