linux-kernel - Re: [PATCH v2 1/6] x86/fpu/xstate: Always preserve non-user xfeatures/flags in __state

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Message-ID: <Z8gKeVN83omngleo@intel.com>
Date: Wed, 5 Mar 2025 16:25:29 +0800
From: Chao Gao <chao.gao@...el.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: <tglx@...utronix.de>, <x86@...nel.org>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<peterz@...radead.org>, <rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>,
	<weijiang.yang@...el.com>, <john.allen@....com>
Subject: Re: [PATCH v2 1/6] x86/fpu/xstate: Always preserve non-user
 xfeatures/flags in __state_perm

On Tue, Mar 04, 2025 at 02:20:31PM -0800, Dave Hansen wrote:
>On 11/26/24 02:17, Chao Gao wrote:
>> When granting userspace or a KVM guest access to an xfeature, preserve the
>> entity's existing supervisor and software-defined permissions as tracked
>> by __state_perm, i.e. use __state_perm to track *all* permissions even
>> though all supported supervisor xfeatures are granted to all FPUs and
>> FPU_GUEST_PERM_LOCKED disallows changing permissions.
>
>Should we document what __state_perm contains either in
>fpu_state_perm or xstate_get_group_perm()?

Yes. I think we should document it. Will apply this change:

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index da6583a1c0a2..93481583dc85 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -417,9 +417,11 @@ struct fpu_state_perm {
 	/*
 	 * @__state_perm:
 	 *
-	 * This bitmap indicates the permission for state components, which
-	 * are available to a thread group. The permission prctl() sets the
-	 * enabled state bits in thread_group_leader()->thread.fpu.
+	 * This bitmap indicates the permission for state components
+	 * available to a thread group, including both user and supervisor
+	 * components and software-defined bits like FPU_GUEST_PERM_LOCKED.
+	 * The permission prctl() sets the enabled state bits in
+	 * thread_group_leader()->thread.fpu.
 	 *
 	 * All run time operations use the per thread information in the
 	 * currently active fpu.fpstate which contains the xfeature masks

>
>Either way:
>
>Acked-by: Dave Hansen <dave.hansen@...el.com>