linux-kernel - Re: [PATCH v10 04/27] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL

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Message-ID: <ZjKNxt1Sq71DI0K8@google.com>
Date: Wed, 1 May 2024 11:45:26 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yang Weijiang <weijiang.yang@...el.com>
Cc: pbonzini@...hat.com, dave.hansen@...el.com, x86@...nel.org, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterz@...radead.org, 
	chao.gao@...el.com, rick.p.edgecombe@...el.com, mlevitsk@...hat.com, 
	john.allen@....com
Subject: Re: [PATCH v10 04/27] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL_DYNAMIC
 xfeature set

On Sun, Feb 18, 2024, Yang Weijiang wrote:
> Define a new XFEATURE_MASK_KERNEL_DYNAMIC mask to specify the features

I still don't understand why this is being called DYNAMIC.  CET_SS isn't dynamic,
as KVM is _always_ allowed to save/restore CET_SS, i.e. whether or not KVM can
expose CET_SS to a guest is a static, boot-time decision.  Whether or not a guest
XSS actually enables CET_SS is "dynamic", but that's true of literally every
xfeature in XCR0 and XSS.

XFEATURE_MASK_XTILE_DATA is labeled as dynamic because userspace has to explicitly
request that XTILE_DATA be enabled, and thus whether or not KVM is allowed to
expose XTILE_DATA to the guest is a dynamic, runtime decision.

So IMO, the umbrella macro should be XFEATURE_MASK_KERNEL_GUEST_ONLY.