From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33003) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eHc5Z-0003dF-D8 for qemu-devel@nongnu.org; Wed, 22 Nov 2017 15:57:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eHc5V-0007PK-RL for qemu-devel@nongnu.org; Wed, 22 Nov 2017 15:57:45 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58440) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eHc5V-0007OW-KW for qemu-devel@nongnu.org; Wed, 22 Nov 2017 15:57:41 -0500 Date: Wed, 22 Nov 2017 21:57:37 +0100 From: Radim =?utf-8?B?S3LEjW3DocWZ?= Message-ID: <20171122205736.GA21184@flask> References: <20171122180908.31389-1-rkrcmar@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH] kvm: apic: save and restore x2APIC LDR List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-devel@nongnu.org, Eduardo Habkost , "Dr . David Alan Gilbert" , Richard Henderson 2017-11-22 20:26+0100, Paolo Bonzini: > On 22/11/2017 19:09, Radim Kr=C4=8Dm=C3=A1=C5=99 wrote: > > 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 > > + if (s->apicbase & MSR_IA32_APICBASE_EXTD) { > > + kvm_apic_set_reg(kapic, 0xd, kvm_apic_calc_x2apic_ldr(s)); >=20 > Is this correct if the kernel doesn't support the new-style x2APIC API? Should be: QEMU will use the APIC_ID register in that case, which contains the x2APIC ID that KVM used to compute the LDR from. (old-style APIC_ID just cannot store more than 8 bits and isn't tied to vcpu_id.) > In the end, it seems simpler to just fix it in the kernel. We already have the workaround in KVM, so dropping this one doesn't make that much of a difference. I perceive it as solely QEMU bug, though. :)