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

[Chao Peng <chao.p.peng@linux.intel.com>]

On Wed, Jul 20, 2022 at 04:44:32PM +0000, Sean Christopherson wrote:
> 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.

It's gfn_t actually. My original purpose for this is 32bit architectures
(if any) can also work with it since index of xarrary is 32bit on those
architectures.  But kvm_enc_region is u64 so itr's even not possible.

> 
> > +	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.

Maybe I missed some information discussed elsewhere? I followed
https://lkml.org/lkml/2022/5/23/772. KVM is shared by default but
userspace should set all guest memory to private before the guest
launch, guest then sees all memory as private.  While default it to
private sounds also good, if we only talk about the private/shared in
private memory context (I think so), then there is no ambiguity.

> 
> 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.

As I think that there may be other state beyond that. Fine with me to
just take consideration of private/shared, and it also sounds
reasonable for people who want to support that to change.

Chao