[PATCH 1/2] ARM: kvm: define PAGE_S2_DEVICE as read-only by default

[Ard Biesheuvel]

On 14 September 2014 11:09, Marc Zyngier <maz at misterjones.org> wrote:
> On 2014-09-14 05:49, Ard Biesheuvel wrote:
>>
>> On 13 September 2014 19:06, Christoffer Dall
>> <christoffer.dall at linaro.org> wrote:
>>>
>>> On Sat, Sep 13, 2014 at 01:15:45PM +0200, Ard Biesheuvel wrote:
>>>>
>>>> On 13 September 2014 12:41, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>>> > Hi Ard,
>>>> >
>>>> > On 2014-09-13 11:17, Ard Biesheuvel wrote:
>>>> >>
>>>> >> Now that we support read-only memslots, we need to make sure that
>>>> >> pass-through device mappings are not mapped writable if the guest
>>>> >> has requested them to be read-only. The existing implementation
>>>> >> already honours this by calling kvm_set_s2pte_writable() on the new
>>>> >> pte in case of writable mappings, so all we need to do is define
>>>> >> the default pgprot_t value used for devices to be PTE_S2_RDONLY.
>>>> >>
>>>> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>>>> >
>>>> >
>>>> > I feel very uncomfortable with this change. Why would we map a device
>>>> > RO? Is
>>>> > that only for completeness sake?
>>>> >
>>>>
>>>> We would map a device RO so that QEMU (or whatever is managing KVM)
>>>> can emulate the writes. I don't have a clear cut use case, to be
>>>> honest, but setting up a writable mapping for a memslot that was
>>>> explicitly set up as read-only seems wrong in any case.
>>>
>>>
>>> Agreed, if it doesn't ever make sense to do so, then we should return an
>>> error to user space if userspace attempts such a configuration.  The
>>> current code is just weird.
>>>
>>>>
>>>> Note that the particular problem I was seeing was primarily caused by
>>>> kvm_is_mmio_pfn()'s false positive on the zero page, but it unveiled
>>>> this particular issue as well.
>>>>
>>>> > Note that we also use PAGE_S2_DEVICE for things that are not mapped
>>>> > through
>>>> > a memslot, such as the GIC.
>>>> >
>>>>
>>>> Yes, and I realize now that this breaks it.
>>>> My apologies: I have an additional patch locally that sets up MMIO
>>>> ranges in one go instead of faulting them in one page at a time as we
>>>> do now, and there the read-write case is handled correctly in
>>>> kvm_phys_addr_ioremap(). However, I thought it was better to send
>>>> these out separately first, but apparently not.
>>>
>>>
>>> I think it is better to change this separately, and then add the ioremap
>>> stuff.  However, you need to change all places that call PAGE_S2_DEVICE
>>> and expect a RDWR memory region, this happens to be only
>>> kvm_phys_addr_ioremap() for now.
>>>
>>>>
>>>> So if we can agree on whether or not MMIO backed mappings should be
>>>> read-write even if the memslot says no, I will follow up with a proper
>>>> series if there are still changes required.
>>>>
>>>
>>> I guess it could be theoretically useful to have read-only device memory
>>> regions, and I can't think of why it would violate the architecture.
>>>
>>
>> We have to handle it either way. But after reading D4.5.3 (Table
>> D4-40) of the ARM ARM, I am wondering why we needed patch b88657674d39
>> "ARM: KVM: user_mem_abort: support stage 2 MMIO page mapping" in the
>> first place, and we could just revert that patch and everything would
>> work as expected. (In short, D4.5.3 says that MT_DEVICE trumps
>> MT_NORMAL, so if the stage 1 translation is MT_DEVICE, it doesn't
>> matter what memtype the S2 translation specifies)
>
>
> We've been there before:
> https://lists.cs.columbia.edu/pipermail/kvmarm/2013-May/004420.html
>

Ah right. So why did those patches not make it in?

Any way, in this case, we have to choose between either dropping the
special case for kvm_is_mmio_pfn(), or fix it honor the writable.
My vote would be to get rid of it.

>>> That said, I don't have any more clear use cases in mind, and we
>>> definitely shouldn't just silently ignore the read-only flag from user
>>> space and make the region writeable.  If we don't want to allow this
>>> behavior, we can return an error in kvm_arch_create_memslot(), which
>>> will cause the KVM_CREATE_USER_MEMORY_REGION ioctl to return -ENOMEM.
>>>
>>
>> Well, I am not sure how easy it is to find out beforehand (i.e., at
>> ioctl time) what the nature of the backing is, and you have to deal
>> with hva_to_pfn() potentially returning a VM_PFNMAP pfn or
>> PageReserved pages anyway.
>> So just mapping everything as MT_NORMAL actually seems like the sanest
>> thing to do, so imo we should revert the patch. The only other
>> question I had is whether it would be better to map a MMIO region in
>> one go, but we can discuss that separately.
>
>
> Aside from the MT_NORMAL thing, the only saving we'd get by dynamically
> maping MMIO regions would be the page tables. Not very useful in my opinion.
>

OK, so you agree faulting it in entirely upon the first abort is a
sane thing to do then.
So 1 patch to change that and 1 to revert the PAGE_S2_DEVICE thing then?