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Date: Wed, 22 Nov 2017 22:01:59 +0100
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] kvm: apic: save and restore x2APIC LDR
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To: Eduardo Habkost <ehabkost@redhat.com>
Cc: qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>, "Dr . David Alan Gilbert" <dgilbert@redhat.com>

2017-11-22 18:28-0200, Eduardo Habkost:
> On Wed, Nov 22, 2017 at 07:09:08PM +0100, Radim Kr=C4=8Dm=C3=A1=C5=99 w=
rote:
> > QEMU saves only 8 bits of APIC LDR, which means that it does not supp=
ort
> > 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.
> >=20
> > KVM always expected the LDR format to follow the xAPIC/x2APIC standar=
d,
> > 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.
> >=20
> > This bug has also been addressed on the KVM side with patch 5849d75a5=
c9b
> > ("KVM: lapic: Fixup LDR on load in x2apic").
>=20
> Is this sufficient to fix the bug on hosts that lack KVM commit
> 5849d75a5c9b, or we need both the KVM and QEMU patches?

Good point, either one is enough to fix the bug.
This patch makes x2APIC LDR work on old KVMs.