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X-CSE-ConnectionGUID: r6EieSdySBCBuQ7AwLhZGg== X-CSE-MsgGUID: EtRwAy/EThSdR9+/5N8Ltg== X-IronPort-AV: E=McAfee;i="6700,10204,11212"; a="14563958" X-IronPort-AV: E=Sophos;i="6.11,167,1725346800"; d="scan'208";a="14563958" Received: from fmviesa002.fm.intel.com ([10.60.135.142]) by fmvoesa110.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Oct 2024 09:12:35 -0700 X-CSE-ConnectionGUID: Ia3wgJFVTL+L5UIycH2/sw== X-CSE-MsgGUID: ezn2VxXjRxmnVOOzGp7JrA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.11,167,1725346800"; d="scan'208";a="97074669" Received: from daliomra-mobl3.amr.corp.intel.com (HELO [10.124.220.1]) ([10.124.220.1]) by fmviesa002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Oct 2024 09:12:33 -0700 Message-ID: <93b3d510-f21b-4f89-ae53-0fa50f03a42d@intel.com> Date: Tue, 1 Oct 2024 09:12:32 -0700 Precedence: bulk X-Mailing-List: linux-doc@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: "Xin Li (Intel)" , 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> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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. 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. 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(): > /* > * 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. :)