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Message-ID: <296f95d8-5506-c95d-1c14-915cbf3e4f30@suse.com> Date: Mon, 21 Nov 2022 18:01:00 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: EFI's -mapbs option may cause Linux to panic() Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Juergen Gross , Andrew Cooper References: <60663f81-09cd-e46b-bb94-1afe50519086@suse.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR3P281CA0038.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:4a::14) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|AS8PR04MB7784:EE_ X-MS-Office365-Filtering-Correlation-Id: 0aaecb1e-c195-45d7-f88c-08dacbe1f853 X-LD-Processed: f7a17af6-1c5c-4a36-aa8b-f5be247aa4ba,ExtFwd X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Then >> Linux fails trying to allocate its lowmem trampoline (which really it >> shouldn't need when running in PV mode), ultimately leading to >> >> panic("Real mode trampoline was not allocated"); >> >> in their init_real_mode(). >> >> While for PV I think it is clear that the easiest is to avoid >> trampoline setup in the first place, iirc PVH Dom0 also tries to >> mirror the host memory map to its own address space. Does PVH Linux >> require a lowmem trampoline? > > Yes, it does AFAIK. I guess those two pages won't be enough for > Linux boot trampoline requirements then. > > I assume native Linux is fine with this memory map because it reclaims > the EfiBootServicesData region and that's enough. That's my understanding as well. >> While the two pages here are just enough for Xen's trampoline, I still >> wonder whether we want to adjust -mapbs behavior. Since whatever we >> might do leaves a risk of conflicting with true firmware (mis)use of >> that space, the best I can think of right now would be another option >> altering behavior (or providing altered behavior). Yet such an option >> would likely need to be more fine-grained then than covering all of >> the low Mb in one go. Which feels like both going too far and making >> it awkward for people to figure out what value(s) to use ... >> >> Thoughts anyone? > > I'm unsure what to recommend. The mapbs option is a workaround for > broken firmware, and it's not enabled by default, so we might be lucky > and never find a system with a memory map like you describe that also > requires mapbs in order to boot. Guess how we've learned of the issue: Systems may boot fine without -mapbs, but they may fail to reboot because of that (in)famous issue of firmware writers not properly separating boot services code paths from runtime services ones. And there we're dealing with a system where I suspect this to be the case, just that - unlike in earlier similar cases - there's no "clean" crash proving the issue (the system simply hangs). Hence my request that they use -mapbs to try to figure out. And yes, "reboot=acpi" helps there, but they insist on knowing what component is to blame. Jan > Any native OS would also have problems booting in such system if it > has any option similar to mapbs, so I don't see much solution. > > Thanks, Roger.