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Message-ID: <4f3a6728-bd4c-715c-a822-4e8f3db447ad@suse.com> Date: Wed, 27 Sep 2023 18:12:06 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v3 7/8] x86: introduce GADDR based secondary time area registration alternative Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , George Dunlap , Julien Grall , Stefano Stabellini , Wei Liu References: <472f8314-9ad7-523a-32dc-d5c2138c2c8c@suse.com> <77218fb0-5e96-4ecb-c2b0-4fe8c3ba683f@suse.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR2P281CA0022.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:14::9) To AS8PR04MB8788.eurprd04.prod.outlook.com (2603:10a6:20b:42f::21) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: AS8PR04MB8788:EE_|PA4PR04MB9712:EE_ X-MS-Office365-Filtering-Correlation-Id: 550d6693-8cde-4d29-76f7-08dbbf747fd4 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Furthermore for 64-bit PV domains the area >> is inaccessible (and hence cannot be updated by Xen) when in guest-user >> mode. >> >> Introduce a new vCPU operation allowing to register the secondary time >> area by guest-physical address. >> >> An at least theoretical downside to using physically registered areas is >> that PV then won't see dirty (and perhaps also accessed) bits set in its >> respective page table entries. >> >> Signed-off-by: Jan Beulich > > Reviewed-by: Roger Pau Monné Thanks. >> --- >> v3: Re-base. >> v2: Forge version in force_update_secondary_system_time(). For your question below, note this revision log entry. I didn't have the compensation originally, and my made-up XTF test thus failed. >> --- a/xen/arch/x86/time.c >> +++ b/xen/arch/x86/time.c >> @@ -1633,6 +1633,16 @@ void force_update_vcpu_system_time(struc >> __update_vcpu_system_time(v, 1); >> } >> >> +void force_update_secondary_system_time(struct vcpu *v, >> + struct vcpu_time_info *map) >> +{ >> + struct vcpu_time_info u; >> + >> + collect_time_info(v, &u); >> + u.version = -1; /* Compensate for version_update_end(). */ > > Hm, we do not seem to compensate in > VCPUOP_register_vcpu_time_memory_area, what's more, in that case the > initial version is picked from the contents of the guest address. > Hopefully the guest will have zeroed the memory. > > FWIW, I would be fine with leaving this at 0, so the first version > guest sees is 1. No, we can't. Odd numbers mean "update in progress". In __update_vcpu_system_time() we properly use version_update_begin(), so there's no need for any compensation. If the guest puts a non-zero value there, that's also fine from the perspective of the protocol. Just that if the initial value is odd, the guest will mislead itself into thinking "oh, the hypervisor is updating this right now" until the first real update has happened. Yet even that is hypothetical, since upon registration the area is first populated, so upon the registration hypercall returning the field will already properly have an even value. Jan