From mboxrd@z Thu Jan 1 00:00:00 1970 From: ard.biesheuvel@linaro.org (Ard Biesheuvel) Date: Sun, 14 Sep 2014 06:49:11 +0200 Subject: [PATCH 1/2] ARM: kvm: define PAGE_S2_DEVICE as read-only by default In-Reply-To: <20140913170638.GA3348@lvm> 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 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) > 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.