On Mon, 26 Sep 2022 11:32:06 -0400 Peter Xu <pet...@redhat.com> wrote:
> It's true that when vcpus<=255 we don't require the length of 32bit APIC > IDs. However here since we already have EIM=ON it means the hypervisor > will declare the VM as x2apic supported (e.g. VT-d ECAP register will have > EIM bit 4 set), so the guest should assume the APIC IDs are 32bits width > even if vcpus<=255. In short, commit 77250171bdc breaks any simple cmdline > that wants to boot a VM with >=9 but <=255 vcpus with: > > -device intel-iommu,intremap=on > > For anyone who does not want to enable x2apic, we can use eim=off in the > intel-iommu parameters to skip enabling KVM x2apic. > > This partly reverts commit 77250171bdc02aee106083fd2a068147befa1a38, while > keeping the valid bit on checking split irqchip, but revert the other change. > > One thing to mention is that this patch may break migration compatibility > of such VM, however that's probably the best thing we can do, because the > old behavior was simply wrong and not working for >8 vcpus. For <=8 vcpus, > there could be a light guest ABI change (by enabling KVM x2apic after this > patch), but logically it shouldn't affect the migration from working. > > Also, this is not the 1st commit to change x2apic behavior. Igor provided > a full history of how this evolved for the past few years: > > https://lore.kernel.org/qemu-devel/20220922154617.57d1a...@redhat.com/ > > Relevant commits for reference: > > fb506e701e ("intel_iommu: reject broken EIM", 2016-10-17) > c1bb5418e3 ("target/i386: Support up to 32768 CPUs without IRQ remapping", > 2020-12-10) > 77250171bd ("intel_iommu: Fix irqchip / X2APIC configuration checks", > 2022-05-16) > dc89f32d92 ("target/i386: Fix sanity check on max APIC ID / X2APIC > enablement", 2022-05-16) > > We may want to have this for stable too (mostly for 7.1.0 only). Adding a > fixes tag. > > Cc: David Woodhouse <dw...@infradead.org> > Cc: Claudio Fontana <cfont...@suse.de> > Cc: Igor Mammedov <imamm...@redhat.com> > Fixes: 77250171bd ("intel_iommu: Fix irqchip / X2APIC configuration checks") > Signed-off-by: Peter Xu <pet...@redhat.com> Reviewed-by: Igor Mammedov <imamm...@redhat.com> > --- > v2: > - Added some more information into commit message [Igor] > --- > hw/i386/intel_iommu.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c > index 05d53a1aa9..6524c2ee32 100644 > --- a/hw/i386/intel_iommu.c > +++ b/hw/i386/intel_iommu.c > @@ -3818,6 +3818,11 @@ static bool vtd_decide_config(IntelIOMMUState *s, > Error **errp) > error_setg(errp, "eim=on requires > accel=kvm,kernel-irqchip=split"); > return false; > } > + if (!kvm_enable_x2apic()) { > + error_setg(errp, "eim=on requires support on the KVM side" > + "(X2APIC_API, first shipped in v4.7)"); > + return false; > + } > } > > /* Currently only address widths supported are 39 and 48 bits */
