[PATCH linux 2/8] xen: introduce xen_vcpu_id mapping

[Vitaly Kuznetsov]

It may happen that Xen's and Linux's ideas of vCPU id diverge. In
particular, when we crash on a secondary vCPU we may want to do kdump
and unlike plain kexec where we do migrate_to_reboot_cpu() we try booting
on the vCPU which crashed. This doesn't work very well for PVHVM guests as
we have a number of hypercalls where we pass vCPU id as a parameter. These
hypercalls either fail or do something unexpected. To solve the issue
introduce percpu xen_vcpu_id mapping. ARM and PV guests get direct mapping
for now. Boot CPU for PVHVM guest gets its id from CPUID. With secondary
CPUs it is a bit more trickier. Currently, we initialize IPI vectors
before these CPUs boot so we can't use CPUID. However, we know that
physical CPU id (vLAPIC id) is Xen's vCPU id * 2, we can piggyback on
that. Alternatively, we could have disabled all secondary CPUs once we
detect that Xen's and Linux's ideas of vCPU id diverged.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 arch/arm/xen/enlighten.c | 10 ++++++++++
 arch/x86/xen/enlighten.c | 18 +++++++++++++++++-
 include/xen/xen-ops.h    |  1 +
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
index 75cd734..ea99ca2 100644
--- a/arch/arm/xen/enlighten.c
+++ b/arch/arm/xen/enlighten.c
@@ -46,6 +46,10 @@ struct shared_info *HYPERVISOR_shared_info = (void *)&xen_dummy_shared_info;
 DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);
 static struct vcpu_info __percpu *xen_vcpu_info;
 
+/* Linux <-> Xen vCPU id mapping */
+DEFINE_PER_CPU(int, xen_vcpu_id) = -1;
+EXPORT_SYMBOL_GPL(xen_vcpu_id);
+
 /* These are unused until we support booting "pre-ballooned" */
 unsigned long xen_released_pages;
 struct xen_memory_region xen_extra_mem[XEN_EXTRA_MEM_MAX_REGIONS] __initdata;
@@ -179,6 +183,9 @@ static void xen_percpu_init(void)
 	pr_info("Xen: initializing cpu%d\n", cpu);
 	vcpup = per_cpu_ptr(xen_vcpu_info, cpu);
 
+	/* Direct vCPU id mapping for ARM guests. */
+	per_cpu(xen_vcpu_id, cpu) = cpu;
+
 	info.mfn = virt_to_gfn(vcpup);
 	info.offset = xen_offset_in_page(vcpup);
 
@@ -328,6 +335,9 @@ static int __init xen_guest_init(void)
 	if (xen_vcpu_info == NULL)
 		return -ENOMEM;
 
+	/* Direct vCPU id mapping for ARM guests. */
+	per_cpu(xen_vcpu_id, 0) = 0;
+
 	if (gnttab_setup_auto_xlat_frames(grant_frames)) {
 		free_percpu(xen_vcpu_info);
 		return -ENOMEM;
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 760789a..69f4c0c 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -59,6 +59,7 @@
 #include <asm/xen/pci.h>
 #include <asm/xen/hypercall.h>
 #include <asm/xen/hypervisor.h>
+#include <asm/xen/cpuid.h>
 #include <asm/fixmap.h>
 #include <asm/processor.h>
 #include <asm/proto.h>
@@ -118,6 +119,10 @@ DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);
  */
 DEFINE_PER_CPU(struct vcpu_info, xen_vcpu_info);
 
+/* Linux <-> Xen vCPU id mapping */
+DEFINE_PER_CPU(int, xen_vcpu_id) = -1;
+EXPORT_SYMBOL_GPL(xen_vcpu_id);
+
 enum xen_domain_type xen_domain_type = XEN_NATIVE;
 EXPORT_SYMBOL_GPL(xen_domain_type);
 
@@ -1137,8 +1142,11 @@ void xen_setup_vcpu_info_placement(void)
 {
 	int cpu;
 
-	for_each_possible_cpu(cpu)
+	for_each_possible_cpu(cpu) {
+		/* Set up direct vCPU id mapping for PV guests. */
+		per_cpu(xen_vcpu_id, cpu) = cpu;
 		xen_vcpu_setup(cpu);
+	}
 
 	/* xen_vcpu_setup managed to place the vcpu_info within the
 	 * percpu area for all cpus, so make use of it. Note that for
@@ -1797,6 +1805,12 @@ static void __init init_hvm_pv_info(void)
 
 	xen_setup_features();
 
+	cpuid(base + 4, &eax, &ebx, &ecx, &edx);
+	if (eax & XEN_HVM_CPUID_VCPU_ID_PRESENT)
+		this_cpu_write(xen_vcpu_id, ebx);
+	else
+		this_cpu_write(xen_vcpu_id, smp_processor_id());
+
 	pv_info.name = "Xen HVM";
 
 	xen_domain_type = XEN_HVM_DOMAIN;
@@ -1808,6 +1822,8 @@ static int xen_hvm_cpu_notify(struct notifier_block *self, unsigned long action,
 	int cpu = (long)hcpu;
 	switch (action) {
 	case CPU_UP_PREPARE:
+		/* vLAPIC_ID == Xen's vCPU_ID * 2 for HVM guests */
+		per_cpu(xen_vcpu_id, cpu) = cpu_physical_id(cpu) / 2;
 		xen_vcpu_setup(cpu);
 		if (xen_have_vector_callback) {
 			if (xen_feature(XENFEAT_hvm_safe_pvclock))
diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
index 86abe07..b02a343 100644
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -8,6 +8,7 @@
 #include <xen/interface/vcpu.h>
 
 DECLARE_PER_CPU(struct vcpu_info *, xen_vcpu);
+DECLARE_PER_CPU(int, xen_vcpu_id);
 
 void xen_arch_pre_suspend(void);
 void xen_arch_post_suspend(int suspend_cancelled);
-- 
2.5.5