Re: [PATCH v10 2/9] KVM: Introduce per-page memory attributes

[Chao Peng]

On Tue, Dec 06, 2022 at 03:07:27PM +0000, Fuad Tabba wrote:
> Hi,
> 
> On Fri, Dec 2, 2022 at 6:18 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > In confidential computing usages, whether a page is private or shared is
> > necessary information for KVM to perform operations like page fault
> > handling, page zapping etc. There are other potential use cases for
> > per-page memory attributes, e.g. to make memory read-only (or no-exec,
> > or exec-only, etc.) without having to modify memslots.
> >
> > Introduce two ioctls (advertised by KVM_CAP_MEMORY_ATTRIBUTES) to allow
> > userspace to operate on the per-page memory attributes.
> >   - KVM_SET_MEMORY_ATTRIBUTES to set the per-page memory attributes to
> >     a guest memory range.
> >   - KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES to return the KVM supported
> >     memory attributes.
> >
> > KVM internally uses xarray to store the per-page memory attributes.
> >
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > Link: https://lore.kernel.org/all/Y2WB48kD0J4VGynX@google.com/
> > ---
> >  Documentation/virt/kvm/api.rst | 63 ++++++++++++++++++++++++++++
> >  arch/x86/kvm/Kconfig           |  1 +
> >  include/linux/kvm_host.h       |  3 ++
> >  include/uapi/linux/kvm.h       | 17 ++++++++
> >  virt/kvm/Kconfig               |  3 ++
> >  virt/kvm/kvm_main.c            | 76 ++++++++++++++++++++++++++++++++++
> >  6 files changed, 163 insertions(+)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 5617bc4f899f..bb2f709c0900 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -5952,6 +5952,59 @@ delivery must be provided via the "reg_aen" struct.
> >  The "pad" and "reserved" fields may be used for future extensions and 
> > should be
> >  set to 0s by userspace.
> >
> > +4.138 KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES
> > +-----------------------------------------
> > +
> > +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> > +:Architectures: x86
> > +:Type: vm ioctl
> > +:Parameters: u64 memory attributes bitmask(out)
> > +:Returns: 0 on success, <0 on error
> > +
> > +Returns supported memory attributes bitmask. Supported memory attributes 
> > will
> > +have the corresponding bits set in u64 memory attributes bitmask.
> > +
> > +The following memory attributes are defined::
> > +
> > +  #define KVM_MEMORY_ATTRIBUTE_READ              (1ULL << 0)
> > +  #define KVM_MEMORY_ATTRIBUTE_WRITE             (1ULL << 1)
> > +  #define KVM_MEMORY_ATTRIBUTE_EXECUTE           (1ULL << 2)
> > +  #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
> > +
> > +4.139 KVM_SET_MEMORY_ATTRIBUTES
> > +-----------------------------------------
> > +
> > +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> > +:Architectures: x86
> > +:Type: vm ioctl
> > +:Parameters: struct kvm_memory_attributes(in/out)
> > +:Returns: 0 on success, <0 on error
> > +
> > +Sets memory attributes for pages in a guest memory range. Parameters are
> > +specified via the following structure::
> > +
> > +  struct kvm_memory_attributes {
> > +       __u64 address;
> > +       __u64 size;
> > +       __u64 attributes;
> > +       __u64 flags;
> > +  };
> > +
> > +The user sets the per-page memory attributes to a guest memory range 
> > indicated
> > +by address/size, and in return KVM adjusts address and size to reflect the
> > +actual pages of the memory range have been successfully set to the 
> > attributes.
> > +If the call returns 0, "address" is updated to the last successful address 
> > + 1
> > +and "size" is updated to the remaining address size that has not been set
> > +successfully. The user should check the return value as well as the size to
> > +decide if the operation succeeded for the whole range or not. The user may 
> > want
> > +to retry the operation with the returned address/size if the previous 
> > range was
> > +partially successful.
> > +
> > +Both address and size should be page aligned and the supported attributes 
> > can be
> > +retrieved with KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES.
> > +
> > +The "flags" field may be used for future extensions and should be set to 
> > 0s.
> > +
> >  5. The kvm_run structure
> >  ========================
> >
> > @@ -8270,6 +8323,16 @@ structure.
> >  When getting the Modified Change Topology Report value, the attr->addr
> >  must point to a byte where the value will be stored or retrieved from.
> >
> > +8.40 KVM_CAP_MEMORY_ATTRIBUTES
> > +------------------------------
> > +
> > +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> > +:Architectures: x86
> > +:Type: vm
> > +
> > +This capability indicates KVM supports per-page memory attributes and 
> > ioctls
> > +KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES/KVM_SET_MEMORY_ATTRIBUTES are 
> > available.
> > +
> >  9. Known KVM API problems
> >  =========================
> >
> > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > index fbeaa9ddef59..a8e379a3afee 100644
> > --- a/arch/x86/kvm/Kconfig
> > +++ b/arch/x86/kvm/Kconfig
> > @@ -49,6 +49,7 @@ config KVM
> >         select SRCU
> >         select INTERVAL_TREE
> >         select HAVE_KVM_PM_NOTIFIER if PM
> > +       select HAVE_KVM_MEMORY_ATTRIBUTES
> >         help
> >           Support hosting fully virtualized guest machines using hardware
> >           virtualization extensions.  You will need a fairly recent
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 8f874a964313..a784e2b06625 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -800,6 +800,9 @@ struct kvm {
> >
> >  #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER
> >         struct notifier_block pm_notifier;
> > +#endif
> > +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> > +       struct xarray mem_attr_array;
> >  #endif
> >         char stats_id[KVM_STATS_NAME_SIZE];
> >  };
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 64dfe9c07c87..5d0941acb5bb 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1182,6 +1182,7 @@ struct kvm_ppc_resize_hpt {
> >  #define KVM_CAP_S390_CPU_TOPOLOGY 222
> >  #define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
> >  #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
> > +#define KVM_CAP_MEMORY_ATTRIBUTES 225
> >
> >  #ifdef KVM_CAP_IRQ_ROUTING
> >
> > @@ -2238,4 +2239,20 @@ struct kvm_s390_zpci_op {
> >  /* flags for kvm_s390_zpci_op->u.reg_aen.flags */
> >  #define KVM_S390_ZPCIOP_REGAEN_HOST    (1 << 0)
> >
> > +/* Available with KVM_CAP_MEMORY_ATTRIBUTES */
> > +#define KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES    _IOR(KVMIO,  0xd2, __u64)
> > +#define KVM_SET_MEMORY_ATTRIBUTES              _IOWR(KVMIO,  0xd3, struct 
> > kvm_memory_attributes)
> > +
> > +struct kvm_memory_attributes {
> > +       __u64 address;
> > +       __u64 size;
> > +       __u64 attributes;
> > +       __u64 flags;
> > +};
> > +
> > +#define KVM_MEMORY_ATTRIBUTE_READ              (1ULL << 0)
> > +#define KVM_MEMORY_ATTRIBUTE_WRITE             (1ULL << 1)
> > +#define KVM_MEMORY_ATTRIBUTE_EXECUTE           (1ULL << 2)
> > +#define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
> 
> nit: how about using the BIT() macro for these?

Might be the _BITULL() in include/uapi/linux/const.h since it will be
used by userspace also.

> 
> > +
> >  #endif /* __LINUX_KVM_H */
> > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> > index 800f9470e36b..effdea5dd4f0 100644
> > --- a/virt/kvm/Kconfig
> > +++ b/virt/kvm/Kconfig
> > @@ -19,6 +19,9 @@ config HAVE_KVM_IRQ_ROUTING
> >  config HAVE_KVM_DIRTY_RING
> >         bool
> >
> > +config HAVE_KVM_MEMORY_ATTRIBUTES
> > +       bool
> > +
> >  # Only strongly ordered architectures can select this, as it doesn't
> >  # put any explicit constraint on userspace ordering. They can also
> >  # select the _ACQ_REL version.
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 1782c4555d94..7f0f5e9f2406 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1150,6 +1150,9 @@ static struct kvm *kvm_create_vm(unsigned long type, 
> > const char *fdname)
> >         spin_lock_init(&kvm->mn_invalidate_lock);
> >         rcuwait_init(&kvm->mn_memslots_update_rcuwait);
> >         xa_init(&kvm->vcpu_array);
> > +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> > +       xa_init(&kvm->mem_attr_array);
> > +#endif
> >
> >         INIT_LIST_HEAD(&kvm->gpc_list);
> >         spin_lock_init(&kvm->gpc_lock);
> > @@ -1323,6 +1326,9 @@ static void kvm_destroy_vm(struct kvm *kvm)
> >                 kvm_free_memslots(kvm, &kvm->__memslots[i][0]);
> >                 kvm_free_memslots(kvm, &kvm->__memslots[i][1]);
> >         }
> > +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> > +       xa_destroy(&kvm->mem_attr_array);
> > +#endif
> >         cleanup_srcu_struct(&kvm->irq_srcu);
> >         cleanup_srcu_struct(&kvm->srcu);
> >         kvm_arch_free_vm(kvm);
> > @@ -2323,6 +2329,49 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm 
> > *kvm,
> >  }
> >  #endif /* CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT */
> >
> > +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> > +static u64 kvm_supported_mem_attributes(struct kvm *kvm)
> > +{
> > +       return 0;
> > +}
> > +
> > +static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> > +                                          struct kvm_memory_attributes 
> > *attrs)
> > +{
> > +       gfn_t start, end;
> > +       unsigned long i;
> > +       void *entry;
> > +       u64 supported_attrs = kvm_supported_mem_attributes(kvm);
> > +
> > +       /* flags is currently not used. */
> 
> nit: "is reserved"? I think it makes it a bit clearer what its purpose is.
OK, then:
  flags is reserved for future extention and currently is not used.

> 
> > +       if (attrs->flags)
> > +               return -EINVAL;
> > +       if (attrs->attributes & ~supported_attrs)
> > +               return -EINVAL;
> > +       if (attrs->size == 0 || attrs->address + attrs->size < 
> > attrs->address)
> > +               return -EINVAL;
> > +       if (!PAGE_ALIGNED(attrs->address) || !PAGE_ALIGNED(attrs->size))
> > +               return -EINVAL;
> > +
> > +       start = attrs->address >> PAGE_SHIFT;
> > +       end = (attrs->address + attrs->size - 1 + PAGE_SIZE) >> PAGE_SHIFT;
> 
> Would using existing helpers be better for getting the frame numbers?

Yes, gpa_to_gfn() can be used.

> Also, the code checks that the address and size are page aligned, so
> the end rounding up seems redundant, and might even be wrong if the
> address+size-1 is close to the gfn_t limit (which this code tries to
> avoid in an earlier check).

That's right.

> 
> > +       entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
> > +
> > +       mutex_lock(&kvm->lock);
> > +       for (i = start; i < end; i++)
> > +               if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> > +                                   GFP_KERNEL_ACCOUNT)))
> > +                       break;
> > +       mutex_unlock(&kvm->lock);
> > +
> > +       attrs->address = i << PAGE_SHIFT;
> > +       attrs->size = (end - i) << PAGE_SHIFT;
> 
> nit: helpers for these too?

Similarly, gfn_to_gpa() will be used.

> 
> With the end calculation fixed,
> 
> Reviewed-by: Fuad Tabba <tabba@google.com>
> After adding the necessary configs for arm64 (on qemu/arm64):
> Tested-by: Fuad Tabba <tabba@google.com>

Thanks.
Chao
> 
> Cheers,
> /fuad
> 
> > +
> > +       return 0;
> > +}
> > +#endif /* CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES */
> > +
> >  struct kvm_memory_slot *gfn_to_memslot(struct kvm *kvm, gfn_t gfn)
> >  {
> >         return __gfn_to_memslot(kvm_memslots(kvm), gfn);
> > @@ -4459,6 +4508,9 @@ static long 
> > kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
> >  #ifdef CONFIG_HAVE_KVM_MSI
> >         case KVM_CAP_SIGNAL_MSI:
> >  #endif
> > +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> > +       case KVM_CAP_MEMORY_ATTRIBUTES:
> > +#endif
> >  #ifdef CONFIG_HAVE_KVM_IRQFD
> >         case KVM_CAP_IRQFD:
> >         case KVM_CAP_IRQFD_RESAMPLE:
> > @@ -4804,6 +4856,30 @@ static long kvm_vm_ioctl(struct file *filp,
> >                 break;
> >         }
> >  #endif /* CONFIG_HAVE_KVM_IRQ_ROUTING */
> > +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> > +       case KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES: {
> > +               u64 attrs = kvm_supported_mem_attributes(kvm);
> > +
> > +               r = -EFAULT;
> > +               if (copy_to_user(argp, &attrs, sizeof(attrs)))
> > +                       goto out;
> > +               r = 0;
> > +               break;
> > +       }
> > +       case KVM_SET_MEMORY_ATTRIBUTES: {
> > +               struct kvm_memory_attributes attrs;
> > +
> > +               r = -EFAULT;
> > +               if (copy_from_user(&attrs, argp, sizeof(attrs)))
> > +                       goto out;
> > +
> > +               r = kvm_vm_ioctl_set_mem_attributes(kvm, &attrs);
> > +
> > +               if (!r && copy_to_user(argp, &attrs, sizeof(attrs)))
> > +                       r = -EFAULT;
> > +               break;
> > +       }
> > +#endif /* CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES */
> >         case KVM_CREATE_DEVICE: {
> >                 struct kvm_create_device cd;
> >
> > --
> > 2.25.1
> >