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

[Raghavendra Rao Ananta]

On Wed, Sep 8, 2021 at 10:09 PM Oliver Upton <oupton@xxxxxxxxxx> wrote:
>
> On Thu, Sep 09, 2021 at 01:38:09AM +0000, Raghavendra Rao Ananta wrote:
> > At times, such as when in the interrupt handler, the guest wants
> > to get the vcpuid that it's running on. As a result, introduce
> > get_vcpuid() that returns the vcpuid of the calling vcpu. At its
> > backend, the VMM prepares a map of vcpuid and mpidr during VM
> > initialization and exports the map to the guest for it to read.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> > ---
> >  .../selftests/kvm/include/aarch64/processor.h |  3 ++
> >  .../selftests/kvm/lib/aarch64/processor.c     | 46 +++++++++++++++++++
> >  2 files changed, 49 insertions(+)
> >
> > diff --git a/tools/testing/selftests/kvm/include/aarch64/processor.h b/tools/testing/selftests/kvm/include/aarch64/processor.h
> > index b6088c3c67a3..150f63101f4c 100644
> > --- a/tools/testing/selftests/kvm/include/aarch64/processor.h
> > +++ b/tools/testing/selftests/kvm/include/aarch64/processor.h
> > @@ -133,6 +133,7 @@ void vm_install_exception_handler(struct kvm_vm *vm,
> >               int vector, handler_fn handler);
> >  void vm_install_sync_handler(struct kvm_vm *vm,
> >               int vector, int ec, handler_fn handler);
> > +void vm_vcpuid_map_init(struct kvm_vm *vm);
> >
> >  static inline void cpu_relax(void)
> >  {
> > @@ -194,4 +195,6 @@ static inline void local_irq_disable(void)
> >       asm volatile("msr daifset, #3" : : : "memory");
> >  }
> >
> > +int get_vcpuid(void);
> > +
>
> I believe both of these functions could use some documentation. The
> former has implicit ordering requirements (can only be called after all
> vCPUs are created) and the latter can only be used within a guest.
>
> >  #endif /* SELFTEST_KVM_PROCESSOR_H */
> > diff --git a/tools/testing/selftests/kvm/lib/aarch64/processor.c b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> > index 632b74d6b3ca..9844b62227b1 100644
> > --- a/tools/testing/selftests/kvm/lib/aarch64/processor.c
> > +++ b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> > @@ -13,9 +13,17 @@
> >  #include "processor.h"
> >
> >  #define DEFAULT_ARM64_GUEST_STACK_VADDR_MIN  0xac0000
> > +#define VM_VCPUID_MAP_INVAL                  -1
> >
> >  static vm_vaddr_t exception_handlers;
> >
> > +struct vm_vcpuid_map {
> > +     uint64_t mpidr;
> > +     int vcpuid;
> > +};
> > +
> > +static struct vm_vcpuid_map vcpuid_map[KVM_MAX_VCPUS];
> > +
>
> Hmm.
>
> I'm not too big of a fan that the KVM_MAX_VCPUS macro is defined in the
> KVM selftests. Really, userspace should discover the limit from the
> kernel. Especially when we want to write tests that test behavior at
> KVM's limit.
>
> That being said, there are more instances of these static allocations in
> the selftests code, so you aren't to be blamed.
>
> Related: commit 074c82c8f7cf ("kvm: x86: Increase MAX_VCPUS to 1024")
> has raised this limit.
>
I'm not a fan of static allocations either, but the fact that
sync_global_to_guest() doesn't have a size argument (yet), makes me
want to take a shorter route. Anyway, if you want I can allocate it
dynamically and copy it to the guest's memory by hand, or come up with
a utility wrapper while I'm at it.
(Just wanted to make sure we are not over-engineering our needs here).

> >  static uint64_t page_align(struct kvm_vm *vm, uint64_t v)
> >  {
> >       return (v + vm->page_size) & ~(vm->page_size - 1);
> > @@ -426,3 +434,41 @@ void vm_install_exception_handler(struct kvm_vm *vm, int vector,
> >       assert(vector < VECTOR_NUM);
> >       handlers->exception_handlers[vector][0] = handler;
> >  }
> > +
> > +void vm_vcpuid_map_init(struct kvm_vm *vm)
> > +{
> > +     int i = 0;
> > +     struct vcpu *vcpu;
> > +     struct vm_vcpuid_map *map;
> > +
> > +     list_for_each_entry(vcpu, &vm->vcpus, list) {
> > +             map = &vcpuid_map[i++];
> > +             map->vcpuid = vcpu->id;
> > +             get_reg(vm, vcpu->id,
> > +                     ARM64_SYS_KVM_REG(SYS_MPIDR_EL1), &map->mpidr);
> > +             map->mpidr &= MPIDR_HWID_BITMASK;
> > +     }
> > +
> > +     if (i < KVM_MAX_VCPUS)
> > +             vcpuid_map[i].vcpuid = VM_VCPUID_MAP_INVAL;
> > +
> > +     sync_global_to_guest(vm, vcpuid_map);
> > +}
> > +
> > +int get_vcpuid(void)
>
> nit: guest_get_vcpuid()
>
Sounds nice. Since we have a lot of guest utility functions now, I'm
fancying a world where we prefix guest_ with all of them to avoid
confusion.

Regards,
Raghavendra
> > +{
> > +     int i, vcpuid;
> > +     uint64_t mpidr = read_sysreg(mpidr_el1) & MPIDR_HWID_BITMASK;
> > +
> > +     for (i = 0; i < KVM_MAX_VCPUS; i++) {
> > +             vcpuid = vcpuid_map[i].vcpuid;
> > +             GUEST_ASSERT_1(vcpuid != VM_VCPUID_MAP_INVAL, mpidr);
> > +
> > +             if (mpidr == vcpuid_map[i].mpidr)
> > +                     return vcpuid;
> > +     }
> > +
> > +     /* We should not be reaching here */
> > +     GUEST_ASSERT_1(0, mpidr);
> > +     return -1;
> > +}
> > --
> > 2.33.0.153.gba50c8fa24-goog
> >