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* [PATCH v4] x86/hyperv: make vapic support x2apic mode
@ 2019-10-10 12:33 Roman Kagan
  0 siblings, 0 replies; only message in thread
From: Roman Kagan @ 2019-10-10 12:33 UTC (permalink / raw)
  To: Michael Kelley, Lan Tianyu, Joerg Roedel, K. Y. Srinivasan,
	Haiyang Zhang, Stephen Hemminger, Sasha Levin, Thomas Gleixner,
	Ingo Molnar, Borislav Petkov, H. Peter Anvin, x86, linux-hyperv,
	linux-kernel, Vitaly Kuznetsov
  Cc: kvm

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@vger.kernel.org
Suggested-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
---
v3 -> v4:
- adjust the log message [Vitaly, Michael]

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 | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
index 5c056b8aebef..e01078e93dd3 100644
--- a/arch/x86/hyperv/hv_apic.c
+++ b/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;
+		}
 	}
 }
-- 
2.21.0


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