On Tue, 20 Jul 2021 12:36:21 +0100, Quentin Perret <qperret@xxxxxxxxxx> wrote: > > On Tuesday 20 Jul 2021 at 12:20:58 (+0100), Marc Zyngier wrote: > > On Tue, 20 Jul 2021 11:38:17 +0100, > > Quentin Perret <qperret@xxxxxxxxxx> wrote: > > > > > > On Tuesday 20 Jul 2021 at 11:21:17 (+0100), Marc Zyngier wrote: > > > > On Tue, 20 Jul 2021 11:09:21 +0100, > > > > Quentin Perret <qperret@xxxxxxxxxx> wrote: > > > > > > > > > > On Thursday 15 Jul 2021 at 17:31:46 (+0100), Marc Zyngier wrote: > > > > > > @@ -815,7 +807,7 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size, > > > > > > .arg = &map_data, > > > > > > }; > > > > > > > > > > > > - if (owner_id > KVM_MAX_OWNER_ID) > > > > > > + if (!annotation || (annotation & PTE_VALID)) > > > > > > return -EINVAL; > > > > > > > > > > Why do you consider annotation==0 invalid? The assumption so far has > > > > > been that the owner_id for the host is 0, so annotating a range with 0s > > > > > should be a valid operation -- this will be required when e.g. > > > > > transferring ownership of a page back to the host. > > > > > > > > How do you then distinguish it from an empty entry that doesn't map to > > > > anything at all? > > > > > > You don't, but that's beauty of it :) > > > > > > The host starts with a PGD full of zeroes, which in terms of ownership > > > means that it owns the entire (I)PA space. And it loses ownership of a > > > page only when we explicitly annotate it with an owner id != 0. > > > > Right. But this scheme doesn't apply to the guests, does it? > > Right, the meaning of a NULL PTE in guests will clearly be something > different, but I guess the interpretation of what invalid mappings mean > is up to the caller. > > > Don't we > > need something that is non-null to preserve the table refcounting? > > Sure, but do we care? If the table entry gets zeroed we're then > basically using an 'invalid block' mapping to annotate the entire block > range with '0', whatever that means. For guests it won't mean much, but > for the host that would mean sole ownership of the entire range. I see. You let the refcount drop to 0, unmap the table and let transfer the 0 annotation one level up, covering the whole block. I guess I'll revert back to allowing 0, but I'd like to make sure we don't do that for guests unless we actually tear down the address space (checking for KVM_PGTABLE_S2_IDMAP should work). Thanks, M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
