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