linux-kernel - Re: [patch v2 21/38] x86/cpu: Provide cpu_init/parse

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Message-ID: <4dba8007-6a37-7ae1-28d2-972aee0a37b1@citrix.com>
Date:   Mon, 31 Jul 2023 15:08:41 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Arjan van de Ven <arjan@...ux.intel.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        Dick Kennedy <dick.kennedy@...adcom.com>,
        James Smart <james.smart@...adcom.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        Jean Delvare <jdelvare@...e.com>,
        Huang Rui <ray.huang@....com>, Juergen Gross <jgross@...e.com>,
        Steve Wahl <steve.wahl@....com>,
        Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>
Subject: Re: [patch v2 21/38] x86/cpu: Provide cpu_init/parse_topology()

On 31/07/2023 2:47 pm, Arjan van de Ven wrote:
> On 7/30/2023 9:05 PM, Michael Kelley (LINUX) wrote:
>> Does anyone have suggestions on a different way to handle
>> this that's better than the above diff?  Other thoughts?
>
> how badly do you need xapic ? Meaning, can x2apic just be used instead
> always

x2APIC under virt is a problem if you don't want to fully emulate an
IOMMU just for int-remapping purposes.

You don't know a-priori whether a particular guest kernel knows about
e.g. the rsvd bit trick in IO-APIC RTEs to allow a 32k destination id.

The only generally compatible way is to start in xAPIC mode, leave all
the enumeration hints around which say "really really please switch into
x2APIC mode", and hope the kernel does.

~Andrew