Roman Kagan <rkagan@xxxxxxxxxxxxx> writes: > Now that there's Hyper-V IOMMU driver, Linux can switch to x2apic mode > when supported by the vcpus. > > However, the apic access functions for Hyper-V enlightened apic assume > xapic mode only. > > As a result, Linux fails to bring up secondary cpus when run as a guest > in QEMU/KVM with both hv_apic and x2apic enabled. > > According to Michael Kelley, when in x2apic mode, the Hyper-V synthetic > apic MSRs behave exactly the same as the corresponding architectural > x2apic MSRs, so there's no need to override the apic accessors. The > only exception is hv_apic_eoi_write, which benefits from lazy EOI when > available; however, its implementation works for both xapic and x2apic > modes. > > Fixes: 29217a474683 ("iommu/hyper-v: Add Hyper-V stub IOMMU driver") > Fixes: 6b48cb5f8347 ("X86/Hyper-V: Enlighten APIC access") > Cc: stable@xxxxxxxxxxxxxxx > Suggested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx> > Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx> > --- > v2 -> v3: > - do not introduce x2apic-capable hv_apic accessors; leave original > x2apic accessors instead > > v1 -> v2: > - add ifdefs to handle !CONFIG_X86_X2APIC > > arch/x86/hyperv/hv_apic.c | 17 +++++++++++++---- > 1 file changed, 13 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c > index 5c056b8aebef..26eeff5bd535 100644 > --- a/arch/x86/hyperv/hv_apic.c > +++ b/arch/x86/hyperv/hv_apic.c > @@ -261,10 +261,19 @@ void __init hv_apic_init(void) > > if (ms_hyperv.hints & HV_X64_APIC_ACCESS_RECOMMENDED) { > pr_info("Hyper-V: Using MSR based APIC access\n"); This pr_info() becomes a bit misleading in x2apic mode, maybe do something like pr_info("Hyper-V: using Enlightened APIC (%s mode)", x2apic_enabled() ? "x2apic" : "xapic"); > + /* > + * With x2apic, architectural x2apic MSRs are equivalent to the > + * respective synthetic MSRs, so there's no need to override > + * the apic accessors. The only exception is > + * hv_apic_eoi_write, because it benefits from lazy EOI when > + * available, but it works for both xapic and x2apic modes. > + */ > apic_set_eoi_write(hv_apic_eoi_write); > - apic->read = hv_apic_read; > - apic->write = hv_apic_write; > - apic->icr_write = hv_apic_icr_write; > - apic->icr_read = hv_apic_icr_read; > + if (!x2apic_enabled()) { > + apic->read = hv_apic_read; > + apic->write = hv_apic_write; > + apic->icr_write = hv_apic_icr_write; > + apic->icr_read = hv_apic_icr_read; > + } > } > } Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> -- Vitaly