On Wed, May 27, 2026 at 07:03:35PM -0500, Michael Roth wrote: > Without in-place conversion, QEMU can still access shared memory to load > initial state into guest memory prior to launch even if the GPA's memory > attributes default to private, since userspace is accessing a completely > separate pool of memory. With in-place conversion, all these accesses > would need to first be converted to shared, then back to private, since > the memory all comes from guest_memfd and only shared memory can be > accessed by userspace. > > To avoid sprinkling these differences in behavior throughout QEMU when > in-place conversion is enabled, just default to shared. This does not > compromise guest security, since Confidential VMs will necessarily > enforce this via trusted entities, and simply generate implicit page > state changes if their default expectations don't match KVM's. However, > in most cases a guest will explicitly convert memory to a particular > state before actually using it, so even these implicit conversion > requests should be rare.
I assume you want to prevent peppering code with convert-in-place conditional paths where, if we default to private, certain mem copies to guest memory would fault. Flip side: we default to shared, we leave the current code paths unchanged (that assume double backing) but we end up (eg on CCA) populating guest memory twice, once with a copy to shared guest memfd memory (that is basically useless because populating in CCA is destructive) and then populating the guest memory region using CCA KVM interface (KVM_ARM_RMI_POPULATE). I am just thinking aloud and sharing what I have noticed so far. Lorenzo > Signed-off-by: Michael Roth <[email protected]> > --- > accel/kvm/kvm-all.c | 21 ++++++++++++++++++++- > 1 file changed, 20 insertions(+), 1 deletion(-) > > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c > index fd01435a0f..c3d399517d 100644 > --- a/accel/kvm/kvm-all.c > +++ b/accel/kvm/kvm-all.c > @@ -1808,7 +1808,26 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml, > abort(); > } > > - if (memory_region_has_guest_memfd(mr)) { > + /* > + * Without in-place conversion, QEMU can still access shared memory > + * to load initial state into guest memory prior to launch even if > + * the GPA's memory attributes default to private, since userspace > + * is accessing a completely separate pool of memory. With in-place > + * conversion, all these accesses would need to first be converted > + * to shared, then back to private, since the memory all comes from > + * guest_memfd and only shared memory can be accessed by userspace. > + * > + * To avoid sprinkling these differences in behavior throughout QEMU > + * when in-place conversion is enabled, just default to shared. This > + * does not compromise guest security, since Confidential VMs will > + * necessarily enforce this via trusted entities, and simply generate > + * implicit page state changes if their default expectations don't > + * match KVM's. However, in most cases a guest will explicitly > + * convert memory to a particular state before actually using it, so > + * even these implicit conversion requests should be rare. > + */ > + if (memory_region_has_guest_memfd(mr) && > + !(current_machine->cgs && > current_machine->cgs->convert_in_place)) { > err = kvm_set_memory_attributes_private(start_addr, slot_size); > if (err) { > error_report("%s: failed to set memory attribute private: > %s", > -- > 2.43.0 >
