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Subject[PATCH v3] x86/hyperv: make vapic support x2apic mode
Date
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>
Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
---
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");
+ /*
+ * 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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