[PATCH] arm/arm64: KVM: Support KVM_CAP_READONLY_MEM

On Fri, Aug 15 2014 at 10:15:50 am BST, Christoffer Dall [off-list ref] wrote:

On Thu, Aug 14, 2014 at 04:46:20PM +0100, Marc Zyngier wrote:

quoted

On Thu, Jul 10 2014 at 3:42:31 pm BST, Christoffer Dall
[off-list ref] wrote:

quoted

When userspace loads code and data in a read-only memory regions, KVM
needs to be able to handle this on arm and arm64.  Specifically this is
used when running code directly from a read-only flash device; the
common scenario is a UEFI blob loaded with the -bios option in QEMU.

To avoid looking through the memslots twice and to reuse the hva error
checking of gfn_to_hva_prot(), add a new gfn_to_hva_memslot_prot()
function and refactor gfn_to_hva_prot() to use this function.

Signed-off-by: Christoffer Dall <redacted>

This looks good to me, but you may want to split the patch in two
(generic stuff, and the ARM code).

sure, I can split it up.

quoted

One question though...

[...]

quoted

quoted

@@ -882,7 +882,10 @@ int kvm_handle_guest_abort(struct kvm_vcpu

*vcpu, struct kvm_run *run)
 	idx = srcu_read_lock(&vcpu->kvm->srcu);
 
 	gfn = fault_ipa >> PAGE_SHIFT;
-	if (!kvm_is_visible_gfn(vcpu->kvm, gfn)) {
+	memslot = gfn_to_memslot(vcpu->kvm, gfn);
+	hva = gfn_to_hva_memslot_prot(memslot, gfn, &writable);
+	write_fault = kvm_is_write_fault(kvm_vcpu_get_hsr(vcpu));
+	if (kvm_is_error_hva(hva) || (write_fault && !writable)) {

So the consequence of a write to a ROM region would be to do an IO
emulation? That seems a bit weird. Shouldn't we have a separate error
path for this (possibly ignoring the write entierely)?

It's part of the ABI, see Documentation/virtual/kvm/api.txt section
4.35:

"The latter [KVM_KVM_READONLY] can be set, if KVM_CAP_READONLY_MEM
capability allows it, to make a new slot read-only.  In this case,
writes to this memory will be posted to userspace as KVM_EXIT_MMIO
exits."

Fair enough. In which case, and assuming you split the patches:

Acked-by: Marc Zyngier <redacted>

	M.
-- 
Jazz is not dead. It just smells funny.