Re: [PATCH v9 4/4] arm: ARMv7 dirty page logging 2nd stage page fault handling support

Mario Smarduch
2014-08-15T00:01:41Z

On 08/13/2014 06:20 PM, Mario Smarduch wrote:
> On 08/13/2014 12:30 AM, Christoffer Dall wrote:
>> On Tue, Aug 12, 2014 at 06:27:11PM -0700, Mario Smarduch wrote:
>>> On 08/12/2014 02:50 AM, Christoffer Dall wrote:
>>>> On Mon, Aug 11, 2014 at 06:25:05PM -0700, Mario Smarduch wrote:
>>>>> On 08/11/2014 12:13 PM, Christoffer Dall wrote:
>>>>>> On Thu, Jul 24, 2014 at 05:56:08PM -0700, Mario Smarduch wrote:
>>
>> [...]
>>
>>>>>>> @@ -1151,7 +1170,7 @@ static void kvm_set_spte_handler(struct kvm *kvm, gpa_t gpa, void *data)
>>>>>>> {
>>>>>>> pte_t *pte = (pte_t *)data;
>>>>>>>
>>>>>>> - stage2_set_pte(kvm, NULL, gpa, pte, false);
>>>>>>> + stage2_set_pte(kvm, NULL, gpa, pte, false, false);
>>>>>>
>>>>>> why is logging never active if we are called from MMU notifiers?
>>>>>
[...]

>> The comment is because when you look at this function it is not obvious
>> why we pass logging_active=false, despite logging may actually be
>> active. This could suggest that the parameter to stage2_set_pte()
>> should be named differently (break_huge_pmds) or something like that,
>> but we can also be satisfied with the comment.
>
> Ok I see, I was thinking you thought it was breaking something.
> Yeah I'll add the comment, in reality this is another use case
> where a PMD may need to be converted to page table so it makes sense
> to contrast use cases.
>
>>
>>>
>>> Should I add comments on flag use in other places as well?
>>>
>>
>> It's always a judgement call. I didn't find it necessarry to put a
>> comment elsewhere because I think it's pretty obivous that we would
>> never care about logging writes to device regions.
>>
>> However, this made me think, are we making sure that we are not marking
>> device mappings as read-only in the wp_range functions? I think it's
>
> KVM_SET_USER_MEMORY_REGION ioctl doesn't check type of region being
> installed/operated on (KVM_MEM_LOG_DIRTY_PAGES), in case of QEMU
> these regions wind up in KVMState->KVMSlot[], when
> memory_region_add_subregion() is called KVM listener installs it.
> For migration and dirty page logging QEMU walks the KVMSlot[] array.
>
> For QEMU VFIO (PCI) mmap()ing BAR of type IORESOURCE_MEM,
> causes the memory region to be added to KVMState->KVMSlot[].
> In that case it's possible to walk KVMState->KVMSlot[] issue
> the ioctl and come across a device mapping with normal memory and
> WP it's s2ptes (VFIO sets unmigrateble state though).
>
> But I'm not sure what's there to stop someone calling the ioctl and
> install a region with device memory type. Most likely though if you
> installed that kind of region migration would be disabled.
>
> But just for logging use not checking memory type could be an issue.

Clarifying above a bit, KVM structures like kvm_run or vgic don't go
through KVM_SET_USER_MEMORY_REGION interface (can't think of any
other KVM structures). VFIO uses KVM_SET_USER_MEMORY_REGION,
user_mem_abort() should resolve the fault. I recall VFIO patch
series add that support.

It should be ok to write protect MMIO regions installed through
KVM_SET_USER_MEMORY_REGION. Although at this time I don't know
of use case for logging without migration, so this may not be
an issue at all at this time.

>
>> quite bad if we mark the VCPU interface as read-only for example.
>>
>> -Christoffer
>>
>