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Re: [Xen-devel] Re: cpuid information to hvm guest

To: "Kamble, Nitin A" <nitin.a.kamble@xxxxxxxxx>
Subject: Re: [Xen-devel] Re: cpuid information to hvm guest
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 29 Sep 2008 22:30:48 +0100
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Thread-topic: [Xen-devel] Re: cpuid information to hvm guest
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How about the following changes to our default HVM CPUID?

 Guest_CPUID.1:EBX[23:17] = Host_CPUID.1:EBX[22:16]
 Guest_CPUID.1:EBX[16] = 0
 Guest_CPUID.4:EAX[31:27] = Host_CPUID.4:EAX[30:26]
 Guest_CPUID.4:EAX[26] = 1
 Guest_CPUID.1:EDX[28] = Host_CPUID.1:EDX[28]

Basically pass-through HT/multicore support, but specify max-cores-per-package and max-threads-per-package as twice the host values. This deals with the fact our virtual APIC IDs are 2*VCPUID.

 -- Keir

On 29/9/08 21:15, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

I’d be happy, by default, for xend to detect threads-per-socket on the host system and replicate that on guest CPUID. I don’t see how direct pass-through of host CPUID values can work, because for example host LAPIC identifiers may be guest different from guest vLAPIC identifiers. So I won’t apply anything like your original patches, period.

 -- Keir

On 29/9/08 18:48, "Kamble, Nitin A" <nitin.a.kamble@xxxxxxxxx> wrote:

Hiding those details by default is a good strategy we’d like to maintain.
[Nitin>] Why do you say it is a good strategy? What are the benefits associated with it?
Furthermore we cannot change CPU-to-LAPIC identifier mapping without breaking saved guest images, which is not acceptable.
[Nitin>]  I am not clear on this part, how saved guest images get broken? I think this change is like upgrading BIOS. It should not break guest images.
If our hiding of host information is broken, we’d like a patch to fix it; likewise any other CPUID inconsistency. What specific issues are you seeing?
[Nitin>] This is an issue with OS which have licensing restriction on CPUs. A customer reported to us that they were not seeing all cpus on windows server because Xen is exposing each vcpu as a socket on a multi-core host system.

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