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

Sean Christopherson
2023-02-14T18:07:51Z

On Mon, Feb 13, 2023, Isaku Yamahata wrote:
> On Fri, Feb 10, 2023 at 12:35:30AM +0000,
> Sean Christopherson <seanjc@google.com> wrote:
>
> > On Wed, Feb 08, 2023, Isaku Yamahata wrote:
> > > On Fri, Dec 02, 2022 at 02:13:40PM +0800,
> > > Chao Peng <chao.p.peng@linux.intel.com> wrote:
> > >
> > > > +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)
> > > > + 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);
> > > > + for (i = start; i < end; i++)
> > > > + if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> > > > + GFP_KERNEL_ACCOUNT)))
> > > > + break;
> > > > + mutex_unlock(&kvm->lock);
> > > > +
> > > > + attrs->address = i << PAGE_SHIFT;
> > > > + attrs->size = (end - i) << PAGE_SHIFT;
> > > > +
> > > > + return 0;
> > > > +}
> > > > +#endif /* CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES */
> > > > +
> > >
> > > If memslot isn't private, it should return error if private attribute is set.
> >
> > Why? I'd rather keep the two things separate. If we enforce this sort of thing
> > at KVM_SET_MEMORY_ATTRIBUTES, then we also have to enforce it at
> > KVM_SET_USER_MEMORY_REGION.
>
> For device assignment via shared GPA, non-private memory slot needs to be
> allowed.

That doesn't say anything about why setting attributes needs to poke into the
memslot. The page fault path already kicks out to userspace if there's a
discrepancy between the attributes and the memslot, why is that insufficient?