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

Sean Christopherson
2023-01-13T22:02:33Z

On Fri, Dec 02, 2022, Chao Peng wrote:
> 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

I would prefer to call this KVM_GENERIC_MEMORY_ATTRIBUTES. Similar to
KVM_GENERIC_HARDWARE_ENABLING, ARM does need/have hardware enabling, it just
doesn't want KVM's generic implementation. In this case, pKVM does support memory
attributes, but uses stage-2 tables to track ownership and doesn't need/want the
overhead of the generic implementation.

> help

...

> +#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)

I think we should carve out bits 0-2 for RWX, but I don't think we should actually
define them until they're actually accepted by KVM.

> +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. */
> + if (attrs->flags)
> + return -EINVAL;
> + if (attrs->attributes & ~supported_attrs)

Nit, no need for "supported_attrs", just consume kvm_supported_mem_attributes()
directly.

> + 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;
> +
> + entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
> +
> + mutex_lock(&kvm->lock);

Peeking forward multiple patches, this needs to take kvm->slots_lock, not kvm->lock.
There's a bug in the lpage_disallowed patch that I believe can most easily be
solved by making this mutually exclusive with memslot changes.

When a memslot is created, KVM needs to walk through the attributes to detect
whether or not the attributes are identical for the entire slot. To avoid races,
that means taking slots_lock.

The alternative would be to query the attributes when adjusting the hugepage level
and avoid lpage_disallowed entirely, but in the (very brief) time I've thought
about this I haven't come up with a way to do that in a performant manner.

> + for (i = start; i < end; i++)

Curly braces needed on the for-loop.