[Xen-devel] [PATCH linux v3 3/9] xen: introduce xen_vcpu_id mapping

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

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. Use ACPI ids from MADT
instead.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
Changes since v2:
- Use uint32_t for xen_vcpu_id mapping [Julien Grall]

Changes since v1:
- Introduce xen_vcpu_nr() helper [David Vrabel]
- Use ACPI ids instead of vLAPIC ids /2 [Andrew Cooper, Jan Beulich]
---
 arch/arm/xen/enlighten.c | 10 ++++++++++
 arch/x86/xen/enlighten.c | 23 ++++++++++++++++++++++-
 include/xen/xen-ops.h    |  6 ++++++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
index 75cd734..fe32267 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(uint32_t, xen_vcpu_id) = U32_MAX;
+EXPORT_PER_CPU_SYMBOL(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 0f87db2..c833912 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(uint32_t, xen_vcpu_id) = U32_MAX;
+EXPORT_PER_CPU_SYMBOL(xen_vcpu_id);
+
 enum xen_domain_type xen_domain_type = XEN_NATIVE;
 EXPORT_SYMBOL_GPL(xen_domain_type);
 
@@ -1135,8 +1140,11 @@ 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
@@ -1727,6 +1735,9 @@ asmlinkage __visible void __init xen_start_kernel(void)
 #endif
        xen_raw_console_write("about to get started...\n");
 
+       /* Let's presume PV guests always boot on vCPU with id 0. */
+       per_cpu(xen_vcpu_id, 0) = 0;
+
        xen_setup_runstate_info(0);
 
        xen_efi_init();
@@ -1795,6 +1806,12 @@ 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;
@@ -1806,6 +1823,10 @@ static int xen_hvm_cpu_notify(struct notifier_block 
*self, unsigned long action,
        int cpu = (long)hcpu;
        switch (action) {
        case CPU_UP_PREPARE:
+               if (cpu_acpi_id(cpu) != U32_MAX)
+                       per_cpu(xen_vcpu_id, cpu) = cpu_acpi_id(cpu);
+               else
+                       per_cpu(xen_vcpu_id, cpu) = cpu;
                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..648ce814 100644
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -9,6 +9,12 @@
 
 DECLARE_PER_CPU(struct vcpu_info *, xen_vcpu);
 
+DECLARE_PER_CPU(uint32_t, xen_vcpu_id);
+static inline int xen_vcpu_nr(int cpu)
+{
+       return per_cpu(xen_vcpu_id, cpu);
+}
+
 void xen_arch_pre_suspend(void);
 void xen_arch_post_suspend(int suspend_cancelled);
 
-- 
2.7.4


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