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Message-ID: Date: Mon, 14 Nov 2022 08:58:28 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.4.2 Subject: Re: [PATCH for-4.17 v2] hvm/apic: repurpose the reporting of the APIC assist options To: Andrew Cooper Cc: "Henry.Wang@arm.com" , Paul Durrant , Wei Liu , Jun Nakajima , Kevin Tian , Roger Pau Monne , "xen-devel@lists.xenproject.org" References: <20221104161815.38007-1-roger.pau@citrix.com> <69841767-ccc4-5eeb-6e9c-d59dfb197ea5@citrix.com> <8e52a065-0141-f75c-03e8-e9ceb18a0aa1@suse.com> <4d0edf44-bb7c-85d9-0890-79c999428ef6@citrix.com> Content-Language: en-US From: Jan Beulich In-Reply-To: <4d0edf44-bb7c-85d9-0890-79c999428ef6@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: AM6P192CA0035.EURP192.PROD.OUTLOOK.COM (2603:10a6:209:83::48) To AM6PR04MB6551.eurprd04.prod.outlook.com (2603:10a6:20b:fa::20) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: AM6PR04MB6551:EE_|AM9PR04MB8356:EE_ X-MS-Office365-Filtering-Correlation-Id: e5342b64-7bbe-4e0d-84d7-08dac616037c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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>>>> if ( viridian_feature_mask(d) & HVMPV_hcall_remote_tlb_flush ) >>>> res->a |= CPUID4A_HCALL_REMOTE_TLB_FLUSH; >>>> - if ( !cpu_has_vmx_apic_reg_virt ) >>>> + if ( !has_assisted_xapic(d) ) >>>> res->a |= CPUID4A_MSR_BASED_APIC; >>> This check is broken before and after.  It needs to be keyed on >>> virtualised interrupt delivery, not register acceleration. >> To me this connection you suggest looks entirely unobvious, so would >> you mind expanding as to why you're thinking so? The hint to the guest >> here is related to how it would best access certain registers (aiui), >> which to me looks orthogonal to how interrupt delivery works. > > I refer you again to the diagram.  (For everyone else on xen-devel, I > put this together when fixing XSA-412 because Intel's APIC acceleration > controls are entirely unintuitive.) > > It is "virtual interrupt delivery" which controls EOI/ICR acceleration, > and not "apic register virtualisation".  There's a decade worth of > hardware where this logic is an anti-optimsiation, by telling windows to > use a VMExit-ing mechanism even when microcode would have avoided the > VMExit. Okay, I agree that I was mislead by the name of the control. Together with Paul (re)clarifying what (virtual) MSRs the CPUID hint is about I agree that it wants to be "virtual interrupt delivery" alone which is checked here. Which in turn makes this a change orthogonal to the other APIC assist work. Jan