linux-kernel - Re: [PATCH 2/2] x86/pkeys: Set XINUSE[PKRU] to 1 so that PKRU is XRSTOR'd correctly

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Message-ID: <0522A1EE-211A-45E1-BAAD-002ABF47D0ED@oracle.com>
Date: Wed, 13 Nov 2024 20:04:51 +0000
From: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo
 Molnar <mingo@...nel.org>,
        Rudi Horn <rudi.horn@...cle.com>, Joe Jin
	<joe.jin@...cle.com>
Subject: Re: [PATCH 2/2] x86/pkeys: Set XINUSE[PKRU] to 1 so that PKRU is
 XRSTOR'd correctly



> On Nov 13, 2024, at 11:10 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 11/13/24 10:14, Aruna Ramakrishna wrote:
>> PKRU value is not XRSTOR'd from the XSAVE area if the corresponding
>> XINUSE[i] bit is 0. When PKRU value is set to 0, it sets XINUSE[PKRU]
>> to 0 on AMD systems, which means the value updated on the sigframe
>> later (after a wrpkru(0)) is ignored.
> 
> I think this is confusing XINUSE and XSTATE_BV.
> 
> XINUSE is really internal to the CPU and is partially exposed in
> xgetbv(1).  But XINUSE is monolithic; it includes user and supervisor state.
> 
> XSTATE_BV is the actual memory location in the XSAVE buffer.
> 
> So I think it's incorrect to say that XRSTOR behavior depends on XINUSE.
> XRSTOR behavior depends on XSTATE_BV.


You’re right; XSTATE_BV[i] is set during XSAVE based on XINUSE[i], and 
XRSTOR restores based on XSTATE_BV. I will reword that (and change the
variable name in update_pkru_in_sigframe()) for v2.

Thanks,
Aruna