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Tue, 1 Apr 2025 17:15:54 +0000 Message-ID: Date: Tue, 1 Apr 2025 10:15:50 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 3/8] x86/fpu/xstate: Add CET supervisor xfeature support To: Chao Gao , , , , , , , CC: , , , , , , Maxim Levitsky , Ingo Molnar , Dave Hansen , "H. Peter Anvin" , Mitchell Levy , "Samuel Holland" , Aruna Ramakrishna , Vignesh Balasubramanian References: <20250318153316.1970147-1-chao.gao@intel.com> <20250318153316.1970147-4-chao.gao@intel.com> Content-Language: en-US From: "Chang S. 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Enabling > this state for host FPUs would lead to a 24-byte waste in the XSAVE buffer > on CET-capable parts. > > Signed-off-by: Yang Weijiang > Signed-off-by: Chao Gao > Reviewed-by: Rick Edgecombe > Reviewed-by: Maxim Levitsky Placing this patch immediately after a few mainline fixes looks to suggest that supervisor CET state can be enabled as-is, implying that the follow-up patches are merely optional optimizations. In V2, Dave provided feedback [1] when you placed this patch second out of six: > This series is starting to look backward to me. > > The normal way you do these things is that you introduce new > abstractions and refactor the code. Then you go adding features. > > For instance, this series should spend a few patches introducing > 'fpu_guest_cfg' and using it before ever introducing the concept of a > dynamic xfeature. In V3, you moved this patch further back to position 8 out of 10. Now, in this version, you've placed it at position 3 out of 8. This raises the question of whether you've fully internalized his advice. If your intent is to save kernel memory, the xstate infrastructure should first be properly adjusted. Specifically: 1. Initialize the VCPU’s default xfeature set and its XSAVE buffer size. 2. Reference them in the two sites: (a) for fpu->guest_perm (b) at VCPU allocation time. 3. Introduce a new feature set (you named "guest supervisor state") as a placeholder and integrate it into initialization, along with the XSAVE sanity check. With these adjustments in place, you may consider enabling a new xfeature, defining it as a guest-supervisor state simply. Thanks, Chang [1] https://lore.kernel.org/kvm/d6127d2e-ea95-4e52-b3db-b39203bad935@intel.com/