linux-kernel - Re: [PATCH v4 09/18] KVM: arm64: selftests: Add guest support to get the vcpuid

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Date:   Fri, 10 Sep 2021 11:03:58 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Andrew Jones <drjones@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v4 09/18] KVM: arm64: selftests: Add guest support to get
 the vcpuid

On Fri, Sep 10, 2021 at 1:10 AM Andrew Jones <drjones@...hat.com> wrote:
>
> On Thu, Sep 09, 2021 at 10:10:56AM -0700, Raghavendra Rao Ananta wrote:
> > On Thu, Sep 9, 2021 at 12:56 AM Andrew Jones <drjones@...hat.com> wrote:
> > >
> > > On Thu, Sep 09, 2021 at 01:38:09AM +0000, Raghavendra Rao Ananta wrote:
> ...
> > > > +     for (i = 0; i < KVM_MAX_VCPUS; i++) {
> > > > +             vcpuid = vcpuid_map[i].vcpuid;
> > > > +             GUEST_ASSERT_1(vcpuid != VM_VCPUID_MAP_INVAL, mpidr);
> > >
> > > We don't want this assert if it's possible to have sparse maps, which
> > > it probably isn't ever going to be, but...
> > >
> > If you look at the way the array is arranged, the element with
> > VM_VCPUID_MAP_INVAL acts as a sentinel for us and all the proper
> > elements would lie before this. So, I don't think we'd have a sparse
> > array here.
>
> If we switch to my suggestion of adding map entries at vcpu-add time and
> removing them at vcpu-rm time, then the array may become sparse depending
> on the order of removals.
>
Oh, I get it now. But like you mentioned, we add entries to the map
while the vCPUs are getting added and then sync_global_to_guest()
later. This seems like a lot of maintainance, unless I'm interpreting
it wrong or not seeing an advantage.
I like your idea of coming up an arch-independent interface, however.
So I modified it similar to the familiar ucall interface that we have
and does everything in one shot to avoid any confusion:

diff --git a/tools/testing/selftests/kvm/include/kvm_util.h
b/tools/testing/selftests/kvm/include/kvm_util.h
index 010b59b13917..0e87cb0c980b 100644
--- a/tools/testing/selftests/kvm/include/kvm_util.h
+++ b/tools/testing/selftests/kvm/include/kvm_util.h
@@ -400,4 +400,24 @@ uint64_t get_ucall(struct kvm_vm *vm, uint32_t
vcpu_id, struct ucall *uc);
 int vm_get_stats_fd(struct kvm_vm *vm);
 int vcpu_get_stats_fd(struct kvm_vm *vm, uint32_t vcpuid);

+#define VM_CPUID_MAP_INVAL -1
+
+struct vm_cpuid_map {
+       uint64_t hw_cpuid;
+       int vcpuid;
+};
+
+/*
+ * Create a vcpuid:hw_cpuid map and export it to the guest
+ *
+ * Input Args:
+ *   vm - KVM VM.
+ *
+ * Output Args: None
+ *
+ * Must be called after all the vCPUs are added to the VM
+ */
+void vm_cpuid_map_init(struct kvm_vm *vm);
+int guest_get_vcpuid(void);
+
 #endif /* SELFTEST_KVM_UTIL_H */
diff --git a/tools/testing/selftests/kvm/lib/aarch64/processor.c
b/tools/testing/selftests/kvm/lib/aarch64/processor.c
index db64ee206064..e796bb3984a6 100644
--- a/tools/testing/selftests/kvm/lib/aarch64/processor.c
+++ b/tools/testing/selftests/kvm/lib/aarch64/processor.c
@@ -16,6 +16,8 @@

 static vm_vaddr_t exception_handlers;

+static struct vm_cpuid_map cpuid_map[KVM_MAX_VCPUS];
+
 static uint64_t page_align(struct kvm_vm *vm, uint64_t v)
 {
        return (v + vm->page_size) & ~(vm->page_size - 1);
@@ -426,3 +428,42 @@ void vm_install_exception_handler(struct kvm_vm
*vm, int vector,
        assert(vector < VECTOR_NUM);
        handlers->exception_handlers[vector][0] = handler;
 }
+
+void vm_cpuid_map_init(struct kvm_vm *vm)
+{
+       int i = 0;
+       struct vcpu *vcpu;
+       struct vm_cpuid_map *map;
+
+       TEST_ASSERT(!list_empty(&vm->vcpus), "vCPUs must have been created\n");
+
+       list_for_each_entry(vcpu, &vm->vcpus, list) {
+               map = &cpuid_map[i++];
+               map->vcpuid = vcpu->id;
+               get_reg(vm, vcpu->id,
KVM_ARM64_SYS_REG(SYS_MPIDR_EL1), &map->hw_cpuid);
+               map->hw_cpuid &= MPIDR_HWID_BITMASK;
+       }
+
+       if (i < KVM_MAX_VCPUS)
+               cpuid_map[i].vcpuid = VM_CPUID_MAP_INVAL;
+
+       sync_global_to_guest(vm, cpuid_map);
+}
+
+int guest_get_vcpuid(void)
+{
+       int i, vcpuid;
+       uint64_t mpidr = read_sysreg(mpidr_el1) & MPIDR_HWID_BITMASK;
+
+       for (i = 0; i < KVM_MAX_VCPUS; i++) {
+               vcpuid = cpuid_map[i].vcpuid;
+
+               /* Was this vCPU added to the VM after the map was
initialized? */
+               GUEST_ASSERT_1(vcpuid != VM_CPUID_MAP_INVAL, mpidr);
+
+               if (mpidr == cpuid_map[i].hw_cpuid)
+                       return vcpuid;
+       }
+
+       /* We should not be reaching here */
+       GUEST_ASSERT_1(0, mpidr);
+       return -1;
+}

This would ensure that we don't have a sparse array and can use the
last non-vCPU element as a sentinal node.
If you still feel preparing the map as and when the vCPUs are created
makes more sense, I can go for it.

Regards,
Raghavendra
> Thanks,
> drew
>