From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marc Zyngier Subject: Re: [PATCH v3 9/9] kvmtool: virtio: enable arm/arm64 support for bi-endianness Date: Wed, 07 May 2014 10:57:01 +0100 Message-ID: <87y4ydoqs2.fsf@approximate.cambridge.arm.com> References: <1398363443-3764-1-git-send-email-marc.zyngier@arm.com> <1398363443-3764-10-git-send-email-marc.zyngier@arm.com> <20140506142807.GI30234@arm.com> <87mweuq0os.fsf@approximate.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain Cc: Peter Maydell , Will Deacon , Pekka Enberg , "kvmarm\@lists.cs.columbia.edu" , "kvm\@vger.kernel.org" , Greg Kurz To: Alexander Graf Return-path: Received: from fw-tnat.austin.arm.com ([217.140.110.23]:27355 "EHLO collaborate-mta1.arm.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754472AbaEGJ5N (ORCPT ); Wed, 7 May 2014 05:57:13 -0400 In-Reply-To: (Alexander Graf's message of "Wed, 7 May 2014 10:42:54 +0100") Sender: kvm-owner@vger.kernel.org List-ID: On Wed, May 07 2014 at 10:42:54 am BST, Alexander Graf wrote: >> Am 07.05.2014 um 11:34 schrieb Peter Maydell : >> >>> On 6 May 2014 19:38, Peter Maydell wrote: >>>> On 6 May 2014 18:25, Marc Zyngier wrote: >>>>> On Tue, May 06 2014 at 3:28:07 pm BST, Will Deacon wrote: >>>>>> On Thu, Apr 24, 2014 at 07:17:23PM +0100, Marc Zyngier wrote: >>>>>> + reg.addr = (u64)&data; >>>>>> + if (ioctl(vcpu->vcpu_fd, KVM_GET_ONE_REG, ®) < 0) >>>>>> + die("KVM_GET_ONE_REG failed (SCTLR_EL1)"); >>>>>> + >>>>>> + return (data & SCTLR_EL1_EE_MASK) ? VIRTIO_ENDIAN_BE : VIRTIO_ENDIAN_LE; >>>>> >>>>> This rules out guests where userspace and kernelspace can run with different >>>>> endinness. Whilst Linux doesn't currently do this, can we support it here? >>>>> It all gets a bit hairy if the guest is using a stage-1 SMMU to let >>>>> userspace play with a virtio device... >>>> >>>> Yeah, I suppose we could check either EE or E0 depending on the mode >>>> when the access was made. We already have all the information, just need >>>> to handle the case. I'll respin the series. >> >>> How virtio implementations should determine their endianness is >>> a spec question, I think; at any rate QEMU and kvmtool ought to >>> agree on how it's done. I think the most recent suggestion on the >>> QEMU mailing list (for PPC) is that we should care about the >>> guest kernel endianness, but I don't know if anybody thought of >>> the pass-through-to-userspace usecase... >> >> Current opinion on the qemu-devel thread seems to be that we >> should just define that the endianness of the virtio device is >> the endianness of the guest kernel at the point where the guest >> triggers a reset of the virtio device by writing zero the QueuePFN >> or Status registers. > > Virtio by design has full access to guest physical memory. It doesn't > route DMA via PCI. So user space drivers simply don't make sense here. Huh? What if my guest has usespace using an idmap, with Stage-1 MMU for isolation only (much like an MPU)? R-class guests anyone? Agreed, this is not the general use case, but that doesn't seem to be completely unrealistic either. M. -- Jazz is not dead. It just smells funny.