Re: [RFC PATCH v11 08/29] KVM: Introduce per-page memory attributes

[Xu Yilun]

On 2023-07-18 at 16:44:51 -0700, Sean Christopherson wrote:
> From: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> 
> 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.
> 
> Use an xarray to store the per-page attributes internally, with a naive,
> not fully optimized implementation, i.e. prioritize correctness over
> performance for the initial implementation.
> 
> Because setting memory attributes is roughly analogous to mprotect() on
> memory that is mapped into the guest, zap existing mappings prior to
> updating the memory attributes.  Opportunistically provide an arch hook
> for the post-set path (needed to complete invalidation anyways) in
> anticipation of x86 needing the hook to update metadata related to
> determining whether or not a given gfn can be backed with various sizes
> of hugepages.
> 
> It's possible that future usages may not require an invalidation, e.g.
> if KVM ends up supporting RWX protections and userspace grants _more_
> protections, but again opt for simplicity and punt optimizations to
> if/when they are needed.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/Y2WB48kD0J4VGynX@xxxxxxxxxx
> Cc: Fuad Tabba <tabba@xxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst |  60 ++++++++++++
>  include/linux/kvm_host.h       |  14 +++
>  include/uapi/linux/kvm.h       |  14 +++
>  virt/kvm/Kconfig               |   4 +
>  virt/kvm/kvm_main.c            | 170 +++++++++++++++++++++++++++++++++
>  5 files changed, 262 insertions(+)
>

Only some trivial concerns below.

[...]
 
> @@ -1175,6 +1176,9 @@ 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_KVM_GENERIC_MEMORY_ATTRIBUTES
> +	xa_init(&kvm->mem_attr_array);
> +#endif
>  
>  	INIT_LIST_HEAD(&kvm->gpc_list);
>  	spin_lock_init(&kvm->gpc_lock);
> @@ -1346,6 +1350,9 @@ 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_KVM_GENERIC_MEMORY_ATTRIBUTES
> +	xa_destroy(&kvm->mem_attr_array);
> +#endif

Is it better to make the destruction in reverse order from the creation?
To put xa_destroy(&kvm->mem_attr_array) after cleanup_srcu_struct(&kvm->srcu),
or put xa_init(&kvm->mem_attr_array) after init_srcu_struct(&kvm->irq_srcu).

>  	cleanup_srcu_struct(&kvm->irq_srcu);
>  	cleanup_srcu_struct(&kvm->srcu);
>  	kvm_arch_free_vm(kvm);
> @@ -2346,6 +2353,145 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
>  }
>  #endif /* CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT */

[...]

> +static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> +					   struct kvm_memory_attributes *attrs)
> +{
> +	gfn_t start, end;
> +
> +	/* flags is currently not used. */
> +	if (attrs->flags)
> +		return -EINVAL;
> +	if (attrs->attributes & ~kvm_supported_mem_attributes(kvm))
> +		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;

As the attrs->address/size are both garanteed to be non-zero, non-wrap
and page aligned in prevous check. Is it OK to simplify the calculation,
like:

  end = (attrs->address + attrs->size) >> PAGE_SHIFT;

> +
> +	if (WARN_ON_ONCE(start == end))
> +		return -EINVAL;

Also, is this check possible to be hit? Maybe remove it?

Thanks,
Yilun

> +
> +	/*
> +	 * xarray tracks data using "unsigned long", and as a result so does
> +	 * KVM.  For simplicity, supports generic attributes only on 64-bit
> +	 * architectures.
> +	 */
> +	BUILD_BUG_ON(sizeof(attrs->attributes) != sizeof(unsigned long));
> +
> +	return kvm_vm_set_mem_attributes(kvm, attrs->attributes, start, end);
> +}
> +#endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */