Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

[Sean Christopherson]

On Wed, Jul 06, 2022, Chao Peng wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 230c8ff9659c..bb714c2a4b06 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -914,6 +914,35 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
>  
>  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
>  
> +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> +#define KVM_MEM_ATTR_PRIVATE 0x0001
> +static int kvm_vm_ioctl_set_encrypted_region(struct kvm *kvm, unsigned int 
> ioctl,
> +                                          struct kvm_enc_region *region)
> +{
> +     unsigned long start, end;

As alluded to in a different reply, because this will track GPAs instead of 
HVAs,
the type needs to be "gpa_t", not "unsigned long".  Oh, actually, they need to
be gfn_t, since those are what gets shoved into the xarray.

> +     void *entry;
> +     int r;
> +
> +     if (region->size == 0 || region->addr + region->size < region->addr)
> +             return -EINVAL;
> +     if (region->addr & (PAGE_SIZE - 1) || region->size & (PAGE_SIZE - 1))
> +             return -EINVAL;
> +
> +     start = region->addr >> PAGE_SHIFT;
> +     end = (region->addr + region->size - 1) >> PAGE_SHIFT;
> +
> +     entry = ioctl == KVM_MEMORY_ENCRYPT_REG_REGION ?
> +                             xa_mk_value(KVM_MEM_ATTR_PRIVATE) : NULL;
> +
> +     r = xa_err(xa_store_range(&kvm->mem_attr_array, start, end,
> +                                     entry, GFP_KERNEL_ACCOUNT));

IIUC, this series treats memory as shared by default.  I think we should invert
that and have KVM's ABI be that all guest memory as private by default, i.e.
require the guest to opt into sharing memory instead of opt out of sharing 
memory.

And then the xarray would track which regions are shared.

Regarding mem_attr_array, it probably makes sense to explicitly include what 
it's
tracking in the name, i.e. name it {private,shared}_mem_array depending on 
whether
it's used to track private vs. shared memory.  If we ever need to track metadata
beyond shared/private then we can tweak the name as needed, e.g. if hardware 
ever
supports secondary non-ephemeral encryption keys.