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Re: [Xen-devel] [PATCH v1] restore the fake x2apic value for cpuid
- To: "Xin Li (Talons)" <xin.li@xxxxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Mon, 3 Dec 2018 12:40:53 +0100
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- Cc: Xin Li <talons.lee@xxxxxxxxx>, Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>, Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Delivery-date: Mon, 03 Dec 2018 11:41:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 03/12/2018 12:15, Xin Li (Talons) wrote:
> Hi Juergen,
> thanks for your comments.
>
> Are you suggesting to call apic_is_x2apic_enabled() in __x2apic_disable()?
> but I think that's exact what changed due to the fake xen_cpuid value.
> Doing so will probably still see the EXTD bit on, and call wrmsrl to
> disable x2apic.
>
> log for linux4.4:
> check_x2apic()
> [ 0.000000] xen_read_msr_safe native read: fee00d00
> [ 0.000000] clear X2APIC_ENABLE
> [ 0.000000] xen_read_msr_safe cpuid: fee00900
> [ 0.000000] x2apic_enabled cpu_has_x2apic: 0 apic_is_x2apic_enabled: 0
>
> log for linux4.19:
> [ 0.000817] xen_read_msr_safe native read: fee00d00
> [ 0.000819] xen_read_msr_safe cpuid: *fee00d00 * //X2APIC_ENABLE not cleared
> [ 0.000820] x2apic_enabled X86_FEATURE_X2APIC: 0 apic_is_x2apic_enabled: 1
Oh, you are right, sorry.
I overlooked rdmsr() is already using xen_read_msr_safe().
OTOH looking into xen_read_msr_safe() I can't see why the X2APIC_ENABLE
bit should be cleared depending on the cpuid value returned. We always
do setup_clear_cpu_cap(X86_FEATURE_X2APIC) for PV domains, so there is
no reason to check the cpuid return data for deciding whether to clear
X2APIC_ENABLE or not. I think we can do that always.
So why don't we remove the #ifdef CONFIG_X86_X2APIC block from
xen_read_msr_safe() ?
Juergen
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