Re: KVM: MMU: nuke shadowed pgtable pages and pte's on memslot destruction

Avi Kivity
2008-07-10T14:42:21Z

Marcelo Tosatti wrote:
> On Mon, Jul 07, 2008 at 02:31:55PM -0300, Marcelo Tosatti wrote:
>
>> On Sun, Jul 06, 2008 at 12:15:56AM +0300, Avi Kivity wrote:
>>
>>> Marcelo Tosatti wrote:
>>>
>>>> On Sat, Jul 05, 2008 at 08:25:30PM +0300, Avi Kivity wrote:
>>>>
>>>>
>>>>>> @@ -1955,6 +1955,22 @@ void kvm_mmu_slot_remove_write_access(st
>>>>>> }
>>>>>> }
>>>>>> +int kvm_mmu_slot_has_shadowed_page(struct kvm *kvm, int slot)
>>>>>> +{
>>>>>> + struct kvm_mmu_page *sp;
>>>>>> + int ret = 0;
>>>>>> +
>>>>>> + spin_lock(&kvm->mmu_lock);
>>>>>> + list_for_each_entry(sp, &kvm->arch.active_mmu_pages, link) {
>>>>>> + if (test_bit(slot, &sp->slot_bitmap)) {
>>>>>> + ret = -EINVAL;
>>>>>> + break;
>>>>>> + }
>>>>>> + }
>>>>>> + spin_unlock(&kvm->mmu_lock);
>>>>>> + return ret;
>>>>>> +}
>>>>>> +
>>>>>>
>>>>>>
>>>>> I don't like the guest influencing host actions in this way. It's
>>>>> just a guest.
>>>>>
>>>>> But I think it's unneeded. kvm_mmu_zap_page() will mark a root
>>>>> shadow page invalid and force all vcpus to reload it, so all that's
>>>>> needed is to keep the mmu spinlock held while removing the slot.
>>>>>
>>>>>
>>>> You're still keeping a shadowed page around with sp->gfn pointing to
>>>> non-existant memslot. The code generally makes the assumption that
>>>> gfn_to_memslot(gfn) on shadowed info will not fail.
>>>>
>>>> kvm_mmu_zap_page -> unaccount_shadowed, for example.
>>>>
>>>>
>>>>
>>> The page has already been zapped, so we might as well
>>> unaccount_shadowed() on the first run. It needs to be moved until after
>>> the reload_remote_mmus() call, though.
>>>
>
> Oops, previous patch was unaccounting multiple times for invalid pages.
> This should be better:
>
>
> During RH6.2 graphical installation the following oops is triggered:
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
> IP: [<ffffffffa00bf172>] :kvm:gfn_to_rmap+0x3e/0x61
> Pid: 4559, comm: qemu-system-x86 Not tainted
>
> The problem is that KVM allows shadow pagetable entries that
> point to a removed memslot to exist. In this case the cirrus vram
> mapping was removed, and the NULL dereference happened during
> kvm_set_memory_alias()'s zap_all_pages().
>
> So nuke all shadowed pages before memslot removal.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
>
> diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
> index a4cf4a2..76259da 100644
> --- a/arch/ia64/kvm/kvm-ia64.c
> +++ b/arch/ia64/kvm/kvm-ia64.c
> @@ -1455,6 +1455,10 @@ int kvm_arch_set_memory_region(struct kvm *kvm,
> return 0;
> }
>
> +int kvm_arch_destroy_memory_region(struct kvm *kvm, int slot)
> +{
> + return 0;
> +}
>
>

This (and its friends) ought to be static inlines.

On the other hand, don't the other arches have to flush their tlbs?
Xiantao/Hollis? So maybe this function needs to be renamed
kvm_flush_shadow() and implemented across the board.

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index b90da0b..5ef3a5e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -405,6 +405,12 @@ int __kvm_set_memory_region(struct kvm *kvm,
> if (mem->slot >= kvm->nmemslots)
> kvm->nmemslots = mem->slot + 1;
>
> + if (!npages) {
> + r = kvm_arch_destroy_memory_region(kvm, mem->slot);
> + if (r)
> + goto out_free;
> + }
> +
>

Destructors should never fail, since there is no possible recovery. And
indeed you have 'return 0' in the actual implementation. So I think the
function better return void.

--
error compiling committee.c: too many arguments to function