Re: [RFC PATCH v12 11/33] KVM: Introduce per-page memory attributes

[Fuad Tabba]

Hi,

> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index d2d913acf0df..f8642ff2eb9d 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1227,6 +1227,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 228
>  #define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 229
>  #define KVM_CAP_USER_MEMORY2 230
> +#define KVM_CAP_MEMORY_ATTRIBUTES 231
>
>  #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -2293,4 +2294,17 @@ 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              _IOW(KVMIO,  0xd3, struct kvm_memory_attributes)
> +
> +struct kvm_memory_attributes {
> +       __u64 address;
> +       __u64 size;
> +       __u64 attributes;
> +       __u64 flags;
> +};
> +
> +#define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
> +

In pKVM, we don't want to allow setting (or clearing) of
PRIVATE/SHARED attributes from userspace. However, we'd like to use
the attributes xarray to track the sharing state of guest pages at the
host kernel.

Moreover, we'd rather the default guest page state be PRIVATE, and
only specify which pages are shared. All pKVM guest pages start off as
private, and the majority will remain so.

I'm not sure if this is the best way to do this: One idea would be to
move the definition of KVM_MEMORY_ATTRIBUTE_PRIVATE to
arch/*/include/asm/kvm_host.h, which is where
kvm_arch_supported_attributes() lives as well. This would allow
different architectures to specify their own attributes (i.e., instead
we'd have a KVM_MEMORY_ATTRIBUTE_SHARED for pKVM). This wouldn't help
in terms of preventing userspace from clearing attributes (i.e.,
setting a 0 attribute) though.

The other thing, which we need for pKVM anyway, is to make
kvm_vm_set_mem_attributes() global, so that it can be called from
outside of kvm_main.c (already have a local patch for this that
declares it in kvm_host.h), and not gate this function by
KVM_GENERIC_MEMORY_ATTRIBUTES. This would let pKVM select only
KVM_PRIVATE_MEM (as opposed to KVM_GENERIC_PRIVATE_MEM, which selects
KVM_GENERIC_MEMORY_ATTRIBUTES), preventing userspace from setting
these attributes, while allowing pKVM to call
kvm_vm_set_mem_attributes().

What do you think?

Thanks,
/fuad