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

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

On 13 September 2014 19:06, Christoffer Dall
<christoffer.dall@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@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@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)

> 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.

-- 
Ard.