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References: <20171122180908.31389-1-rkrcmar@redhat.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <adada925-d23d-af1c-8965-f18904f5184d@redhat.com>
Date: Wed, 22 Nov 2017 20:26:14 +0100
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Subject: Re: [Qemu-devel] [PATCH] kvm: apic: save and restore x2APIC LDR
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To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>, qemu-devel@nongnu.org
Cc: Eduardo Habkost <ehabkost@redhat.com>, "Dr . David Alan Gilbert" <dgilbert@redhat.com>, Richard Henderson <rth@twiddle.net>

On 22/11/2017 19:09, Radim Krčmář wrote:
> QEMU saves only 8 bits of APIC LDR, which means that it does not support
> x2APIC.  The correct way of fixing this would be to save and restore the
> full 32 bit register, but because x2APIC LDR is a function of x2APIC ID,
> we can also compute it and keep the migration format untouched.
> 
> KVM always expected the LDR format to follow the xAPIC/x2APIC standard,
> but pre 4.1 KVMs used non-standard x2APIC ID in case the OS changed
> xAPIC ID before switching to x2APIC, which means that QEMU has to use
> the kvm_x2apic_api feature to derive the x2APIC ID.
> 
> This bug has also been addressed on the KVM side with patch 5849d75a5c9b
> ("KVM: lapic: Fixup LDR on load in x2apic").

> +    if (s->apicbase & MSR_IA32_APICBASE_EXTD) {
> +        kvm_apic_set_reg(kapic, 0xd, kvm_apic_calc_x2apic_ldr(s));

Is this correct if the kernel doesn't support the new-style x2APIC API?

In the end, it seems simpler to just fix it in the kernel.

Paolo

> +    } else {
> +        kvm_apic_set_reg(kapic, 0xd, s->log_dest << 24);
> +    }