x86/hyperv: Make vapic support x2apic mode

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: 29217a4 ("iommu/hyper-v: Add Hyper-V stub IOMMU driver")
Fixes: 6b48cb5 ("X86/Hyper-V: Enlighten APIC access")
Suggested-by: Michael Kelley <[email protected]>
Signed-off-by: Roman Kagan <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Vitaly Kuznetsov <[email protected]>
Reviewed-by: Michael Kelley <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

Author: rvka

arch/x86/hyperv/hv_apic.c

@@ -260,11 +260,21 @@ 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");
+		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;
+		}
 	}
 }