From mboxrd@z Thu Jan 1 00:00:00 1970 From: ard.biesheuvel@linaro.org (Ard Biesheuvel) Date: Sun, 14 Sep 2014 11:43:39 +0200 Subject: [PATCH 1/2] ARM: kvm: define PAGE_S2_DEVICE as read-only by default In-Reply-To: References: <1410603462-28900-1-git-send-email-ard.biesheuvel@linaro.org> <20140913170638.GA3348@lvm> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 14 September 2014 11:09, Marc Zyngier wrote: > On 2014-09-14 05:49, Ard Biesheuvel wrote: >> >> On 13 September 2014 19:06, Christoffer Dall >> wrote: >>> >>> On Sat, Sep 13, 2014 at 01:15:45PM +0200, Ard Biesheuvel wrote: >>>> >>>> On 13 September 2014 12:41, Marc Zyngier 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 >>>> > >>>> > >>>> > 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?