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Message-ID: <87shvwur5p.fsf@vitty.brq.redhat.com>
Date: Wed, 29 Jun 2016 14:16:02 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: <xen-devel@...ts.xenproject.org>, Juergen Gross <jgross@...e.com>,
Stefano Stabellini <sstabellini@...nel.org>, <x86@...nel.org>,
<linux-kernel@...r.kernel.org>,
Julien Grall <julien.grall@...rix.com>,
Ingo Molnar <mingo@...hat.com>,
"David Vrabel" <david.vrabel@...rix.com>,
"H. Peter Anvin" <hpa@...or.com>,
"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping
Andrew Cooper <andrew.cooper3@...rix.com> writes:
> On 28/06/16 17:47, Vitaly Kuznetsov wrote:
>> @@ -1808,6 +1822,8 @@ static int xen_hvm_cpu_notify(struct notifier_block *self, unsigned long action,
>> int cpu = (long)hcpu;
>> switch (action) {
>> case CPU_UP_PREPARE:
>> + /* vLAPIC_ID == Xen's vCPU_ID * 2 for HVM guests */
>> + per_cpu(xen_vcpu_id, cpu) = cpu_physical_id(cpu) / 2;
>
> Please do not assume or propagate this brokenness. It is incorrect in
> the general case, and I will be fixing in the hypervisor in due course.
>
> Always read the APIC_ID from the LAPIC, per regular hardware.
(I'm probbaly missing something important - please bear with me)
The problem here is that I need to get _other_ CPU's id before any code
is executed on that CPU (or, at least, this is the current state of
affairs if you look at xen_hvm_cpu_up()) so I can't use CPUID/do MSR
reads/... The only option I see here is to rely on ACPI (MADT) data
which is stored in x86_cpu_to_apicid (and that's what cpu_physical_id()
gives us). MADT also has processor id which connects it to DSDT but I'm
not sure Linux keeps this data. But this is something fixable I guess.
--
Vitaly
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