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: <5a98b9bf-8004-47e0-82e6-77da49419e0b@intel.com>
Date: Wed, 13 Nov 2024 11:10:01 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>,
 linux-kernel@...r.kernel.org
Cc: x86@...nel.org, dave.hansen@...ux.intel.com, tglx@...utronix.de,
 mingo@...nel.org, rudi.horn@...cle.com, joe.jin@...cle.com
Subject: Re: [PATCH 2/2] x86/pkeys: Set XINUSE[PKRU] to 1 so that PKRU is
 XRSTOR'd correctly

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.