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Wed, 2 Apr 2025 02:28:29 +0000 Date: Wed, 2 Apr 2025 10:28:16 +0800 From: Chao Gao To: "Chang S. Bae" CC: , , , , , , , , , , , , , Maxim Levitsky , Ingo Molnar , Dave Hansen , "H. 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Bae wrote: >On 3/18/2025 8:31 AM, Chao Gao wrote: >> From: Yang Weijiang >> >> To support CET virtualization, KVM needs the kernel to save and restore >> the CET supervisor xstate in guest FPUs when switching between guest and >> host FPUs. >> >> Add CET supervisor xstate support in preparation for the upcoming CET >> virtualization in KVM. >> >> Currently, host FPUs do not utilize the CET supervisor xstate. 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. Yes, this is intentional. I mentioned it in the cover letter: """ Reorder the patches to put the CET supervisor state patch before the "guest-only" optimization, allowing maintainers to easily adopt or omit the optimization. """ > >In V2, Dave provided feedback [1] when you placed this patch second out of >six: In my opinion, he wasn't referring to the patch introducing the CET supervisor xstate (i.e., this patch). Rather, he requested that the patch making the CET supervisor xstate a guest-only feature should follow the introduction of fpu_guest_cfg and the relevant cleanups. > > > 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. I believe you are suggesting that the CET supervisor xstate should be introduced directly as a guest-only feature, rather than first introducing it in one patch and then converting it to guest-only in a subsequent patch. This is a valid point, and I have considered it. However, I chose to split them into two patches because the guest-only aspect is merely an optimization, and the decision on whether to accept it is still pending. This order and split-up make it easier for maintainers to make a decision.