From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: mx.groups.io; dkim=missing; spf=pass (domain: redhat.com, ip: 209.132.183.28, mailfrom: lersek@redhat.com) Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by groups.io with SMTP; Mon, 26 Aug 2019 08:30:48 -0700 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id C996F307D915; Mon, 26 Aug 2019 15:30:47 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-117-34.ams2.redhat.com [10.36.117.34]) by smtp.corp.redhat.com (Postfix) with ESMTP id C05391001944; Mon, 26 Aug 2019 15:30:44 +0000 (UTC) Subject: Re: [edk2-rfc] [edk2-devel] CPU hotplug using SMM with QEMU+OVMF To: "Kinney, Michael D" , "Yao, Jiewen" , Paolo Bonzini , "rfc@edk2.groups.io" Cc: Alex Williamson , "devel@edk2.groups.io" , qemu devel list , Igor Mammedov , "Chen, Yingwen" , "Nakajima, Jun" , Boris Ostrovsky , Joao Marcal Lemos Martins , Phillip Goerl References: <8091f6e8-b1ec-f017-1430-00b0255729f4@redhat.com> <20190816161933.7d30a881@x1.home> <74D8A39837DF1E4DA445A8C0B3885C503F761B96@shsmsx102.ccr.corp.intel.com> <35396800-32d2-c25f-b0d0-2d7cd8438687@redhat.com> <2b4ba607-f0e3-efee-6712-6dcef129b310@redhat.com> <7f2d2f1e-2dd8-6914-c55e-61067e06b142@redhat.com> <3661c0c5-3da4-1453-a66a-3e4d4022e876@redhat.com> <74D8A39837DF1E4DA445A8C0B3885C503F76FDAF@shsmsx102.ccr.corp.intel.com> From: "Laszlo Ersek" Message-ID: Date: Mon, 26 Aug 2019 17:30:43 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Mon, 26 Aug 2019 15:30:47 +0000 (UTC) Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 08/23/19 17:25, Kinney, Michael D wrote: > Hi Jiewen, > > If a hot add CPU needs to run any code before the > first SMI, I would recommend is only executes code > from a write protected FLASH range without a stack > and then wait for the first SMI. "without a stack" looks very risky to me. Even if we manage to implement the guest code initially, we'll be trapped without a stack, should we ever need to add more complex stuff there. > For this OVMF use case, is any CPU init required > before the first SMI? I expressed a preference for that too: "I wish we could simply wake the new CPU [...] with an SMI". http://mid.mail-archive.com/398b3327-0820-95af-a34d-1a4a1d50cf35@redhat.com > From Paolo's list of steps are steps (8a) and (8b) > really required? See again my message linked above -- just after the quoted sentence, I wrote, "IOW, if we could excise steps 07b, 08a, 08b". But, I obviously defer to Paolo and Igor on that. (I do believe we have a dilemma here. In QEMU, we probably prefer to emulate physical hardware as faithfully as possible. However, we do not have Cache-As-RAM (nor do we intend to, IIUC). Does that justify other divergences from physical hardware too, such as waking just by virtue of an SMI?) > Can the SMI monarch use the Local > APIC to send a directed SMI to the hot added CPU? > The SMI monarch needs to know the APIC ID of the > hot added CPU. Do we also need to handle the case > where multiple CPUs are added at once? I think we > would need to serialize the use of 3000:8000 for the > SMM rebase operation on each hot added CPU. I agree this would be a huge help. > It would be simpler if we can guarantee that only > one CPU can be added or removed at a time and the > complete flow of adding a CPU to SMM and the OS > needs to be completed before another add/remove > event needs to be processed. I don't know if the QEMU monitor command in question can guarantee this serialization. I think such a request/response pattern is generally implementable between QEMU and guest code. But, AIUI, the "device-add" monitor command is quite generic, and used for hot-plugging a number of other (non-CPU) device models. I'm unsure if the pattern in question can be squeezed into "device-add". (It's not a dedicated command for CPU hotplug.) ... Apologies that I didn't add much information to the thread, just now. I'd like to keep the discussion going. Thanks Laszlo