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Message-ID: <3f79d18f-8289-87df-1bbb-336fac5f381e@suse.com> Date: Thu, 20 Apr 2023 16:55:32 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v6] x86: detect CMOS aliasing on ports other than 0x70/0x71 Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Paul Durrant , Wei Liu References: <5426dd6f-50cd-dc23-5c6b-0ab631d98d38@suse.com> <116bb94f-955c-4c46-f16a-d7a5e1cc72b5@suse.com> <7e3246c7-6de2-b3eb-477f-99ef9bd1b33b@suse.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR2P281CA0050.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:92::13) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|AS8PR04MB8641:EE_ X-MS-Office365-Filtering-Correlation-Id: 2e14d2ae-f43a-473e-999b-08db41af4b6f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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>>>> + >>>> + /* >>>> + * When there's (supposedly) no RTC/CMOS, we don't intercept the other >>>> + * ports. While reading the index register isn't normally possible, >>>> + * play safe and return back whatever can be read (just in case a value >>>> + * written through an alias would be attempted to be read back here). >>>> + */ >>>> + if ( port == RTC_PORT(0) && >>>> + (acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_CMOS_RTC) && >>>> + ioports_access_permitted(currd, port, port) ) >>>> + data = inb(port) & 0x7f; >>> >>> Do we really need to mask the high bit here? We don't allow setting >>> that bit in the first place. >> >> I think it's more consistent to mask it off, specifically with the code >> visible in context right above the insertion. The doc isn't really clear >> about readability of that bit: On one hand in says R/W for port 0x70 in >> the NMI_EN section, yet otoh in the RTC section it says "Note that port >> 70h is not directly readable. The only way to read this register is >> through Alt Access mode." (I think the NMI_EN section is more trustworthy, >> but still.) Plus if we were to ever make use of the NMI disable, we >> wouldn't want Dom0 see the bit set. > > I guess so, at the end Xen itself doesn't use the bit so far. Maybe > at some point we would want to expose the value of the bit to dom0 if > Xen starts using it (most than anything for informative purposes if > NMIs are disabled). > > Feel free to fold the diff to the existing patch and keep the RB. Thanks. > I guess you will also add something to the commit message about the > special handling of the NMI enable bit even when the RTC/CMOS is not > present? Of course, albeit not more than a sentence, as the code comments provide the details. Jan