From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 399C715E97; Tue, 1 Oct 2024 17:52:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727805144; cv=none; b=TEbNyVs9/OyANVKf+cAogP1N1vL2eeQzR21xLfPtBcJRggzNaukklVH0Sp0GPgVZzWK3E34Qb8AvRSXnsaERBNFyVQAco6VARJWmEPKxJqRQNYab0Ej+e2GKu0B3TwsW4b0GiAP0K8OCzE7JI5Og5aZTJ5gyKPCO5gBZiZQOKiE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727805144; c=relaxed/simple; bh=E/3fGNUQcLORvx8K8F33lG493tdBqd3VfuxRHO04oDE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Nipf1x+ude3eLbI86WHrrWw9lgIp1Nybhhru2VCScuRpSX5eER4Vb1yY9qLBX5Upy5ZaU/8JtcxDHrpUb6yUb5vwsZ0uWHKhM9OHW8Na604f/TSRmWwOeZoruEmHWIwn7dsWlcIi/pPErkcRi8F6Sj8JRZz2btXu4mUSOaAbJck= ARC-Authentication-Results:i=1; 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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=TST/sOe1en6bYGqJ0ofmbhaOyik+Cw5k+AESQgkmRTYYoUb5aW+4hYADgivwI8S+q oeowugrgwR8uVgCMofB5f/mbIUNYgy7vrP1o/1ukVVGZ0RPhfspYUGKPhvl38TAYWa M6s6p8qQ/J/FqntpqIGsvbYn32Vfsd0aVy7zjD6h2oJbRtOusYgYWnGCQ0pJQCtkv2 LQULRMLSv/pvlfpzN6QtiQgtZzlBTLQrUWTMcjpCdA7dTya4oiUkUf4QNsq+qq3M/A 5Cejib9G94caY49Cx2CF7VDmvfitm6Skb+fFU/ggjqXd60cgDbs2hEWerYaQLRHMf7 gogrxZOgHIG6Q== Message-ID: Date: Tue, 1 Oct 2024 10:51:32 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 06/27] x86/cea: Export per CPU variable cea_exception_stacks To: Dave Hansen , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Cc: seanjc@google.com, pbonzini@redhat.com, corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, luto@kernel.org, peterz@infradead.org, andrew.cooper3@citrix.com References: <20241001050110.3643764-1-xin@zytor.com> <20241001050110.3643764-7-xin@zytor.com> <93b3d510-f21b-4f89-ae53-0fa50f03a42d@intel.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/1/2024 9:12 AM, Dave Hansen wrote: > On 9/30/24 22:00, Xin Li (Intel) wrote: >> The per CPU variable cea_exception_stacks contains per CPU stacks for >> NMI, #DB and #DF, which is referenced in KVM to set host FRED RSP[123] >> each time a vCPU is loaded onto a CPU, thus it needs to be exported. > > Nit: It's not obvious how 'cea_exception_stacks' get used in this > series. It's never referenced explicitly. > > I did figure it out by looking for 'RSP[123]' references, but a much > better changelog would be something like: > > The per CPU array 'cea_exception_stacks' points to per CPU > stacks for NMI, #DB and #DF. It is normally referenced via the > #define: __this_cpu_ist_top_va(). > > FRED introduced new fields in the host-state area of the VMCS > for stack levels 1->3 (HOST_IA32_FRED_RSP[123]). KVM must > populate these each time a vCPU is loaded onto a CPU. > Yeah, this is way clearer. > See how that explicitly gives the reader greppable strings for > "__this_cpu_ist_top_va" and "HOST_IA32_FRED_RSP"? That makes it much > easier to figure out what is going on. Nice for a maintainer in 20 years :) > > I was also momentarily confused about why these loads need to be done on > _every_ vCPU load. I think it's because the host state can change as > the vCPU moves around to different physical CPUs and > __this_cpu_ist_top_va() can and will change. But it's a detail that I > think deserves to be explained in the changelog. There is also this > note in vmx_vcpu_load_vmcs(): Makes sense to me. > >> /* >> * Linux uses per-cpu TSS and GDT, so set these when switching >> * processors. See 22.2.4. >> */ > > which makes me think that it might not be bad to pull *all* of the > per-cpu VMCS field population code out into a helper since the reasoning > of why these need to be repopulated is identical. > > Also, what's the purpose of clearing GUEST_IA32_FRED_RSP[123] at > init_vmcs() time? I would have thought that those values wouldn't > matter until the VMCS gets loaded at vmx_vcpu_load_vmcs() when they are > overwritten anyway. Or, I could be just totally misunderstanding how > KVM consumes the VMCS. :) I don't see any misunderstanding. However we just do what the SDM claims, even it seems that it's not a must *logically*. FRED spec says: The RESET state of each of the new MSRs is zero. INIT does not change the value of the new MSRs Thanks! Xin