On Wed, Dec 03, 2014 at 10:47:34AM +0000, Andre Przywara wrote: > > > On 03/12/14 10:30, Christoffer Dall wrote: > > On Tue, Dec 02, 2014 at 05:32:45PM +0000, Andre Przywara wrote: > >> On 02/12/14 17:06, Marc Zyngier wrote: > >>> On 02/12/14 16:24, Andre Przywara wrote: > >>>> Hej Christoffer, > >>>> > >>>> On 30/11/14 08:30, Christoffer Dall wrote: > >>>>> On Fri, Nov 28, 2014 at 03:24:11PM +0000, Andre Przywara wrote: > >>>>>> Hej Christoffer, > >>>>>> > >>>>>> On 25/11/14 10:41, Christoffer Dall wrote: > >>>>>>> Hi Andre, > >>>>>>> > >>>>>>> On Mon, Nov 24, 2014 at 04:00:46PM +0000, Andre Przywara wrote: > >>>>>>> > >>>>>> > >>>> > >>>> [...] > >>>> > >>>>>>>>>> + > >>>>>>>>>> + if (!is_in_range(mmio->phys_addr, mmio->len, rdbase, > >>>>>>>>>> + GIC_V3_REDIST_SIZE * nrcpus)) > >>>>>>>>>> + return false; > >>>>>>>>> > >>>>>>>>> Did you think more about the contiguous allocation issue here or can you > >>>>>>>>> give me a pointer to the requirement in the spec? > >>>>>>>> > >>>>>>>> 5.4.1 Re-Distributor Addressing > >>>>>>>> > >>>>>>> > >>>>>>> Section 5.4.1 talks about the pages within a single re-distributor having > >>>>>>> to be contiguous, not all the re-deistributor regions having to be > >>>>>>> contiguous, right? > >>>>>> > >>>>>> Ah yes, you are right. But I still think it does not matter: > >>>>>> 1) We are "implementing" the GICv3. So as the spec does not forbid this, > >>>>>> we just state that the redistributor register maps for each VCPU are > >>>>>> contiguous. Also we create the FDT accordingly. I will add a comment in > >>>>>> the documentation to state this. > >>>>>> > >>>>>> 2) The kernel's GICv3 DT bindings assume this allocation is the default. > >>>>>> Although Marc added bindings to work around this (stride), it seems much > >>>>>> more logical to me to not use it. > >>>>> > >>>>> I don't disagree (and never have) with the fact that it is up to us to > >>>>> decide. > >>>>> > >>>>> My original question, which we haven't talked about yet, is if it is > >>>>> *reasonable* to assume that all re-distributor regions will always be > >>>>> contiguous? > >>>>> > >>>>> How will you handle VCPU hotplug for example? > >>>> > >>>> As kvmtool does not support hotplug, I haven't thought about this yet. > >>>> To me it looks like userland should just use maxcpus for the allocation. > >>>> If I get the current QEMU code right, there is room for 127 GICv3 VCPUs > >>>> (2*64K per VCPU + 64K for the distributor in 16M space) at the moment. > >>>> Kvmtool uses a different mapping, which allows to share 1G with virtio, > >>>> so the limit is around 8000ish VCPUs here. > >>>> Are there any issues with changing the QEMU virt mapping later? > >>>> Migration, maybe? > >>>> If the UART, the RTC and the virtio regions are moved more towards the > >>>> beginning of the 256MB PCI mapping, then there should be space for a bit > >>>> less than 1024 VCPUs, if I get this right. > >>>> > >>>>> Where in the guest > >>>>> physical memory map of our various virt machines should these regions > >>>>> sit so that we can allocate anough re-distributors for VCPUs etc.? > >>>> > >>>> Various? Are there other mappings than those described in hw/arm/virt.c? > >>>> > >>>>> I just want to make sure we're not limiting ourselves by some amount of > >>>>> functionality or ABI (redistributor base addresses) that will be hard to > >>>>> expand in the future. > >>>> > >>>> If we are flexible with the mapping at VM creation time, QEMU could just > >>>> use a mapping depending on max_cpus: > >>>> < 128 VCPUs: use the current mapping > >>>> 128 <= x < 1020: use a more compressed mapping > >>>>> = 1020: map the redistributor somewhere above 4 GB > >>>> > >>>> As the device tree binding for GICv3 just supports a stride value, we > >>>> don't have any other real options beside this, right? So how I see this, > >>>> a contiguous mapping (with possible holes) is the only way. > >>> > >>> Not really. The GICv3 binding definitely supports having several regions > >>> for the redistributors (see the binding documentation). This allows for > >>> the pathological case where you have N regions for N CPUs. Not that we > >>> ever want to go there, really. > >> > >> Ah yes, thanks for pointing that out. I was mixing this up with the > >> stride parameter, which is independent of this. Sorry for that. > >> > >> So from a userland point of view we probably would like to have the > >> first n VCPU's redistributors mapped at their current places and allow > >> for more VCPUs to use memory above 4 GB. > >> Which would require quite some changes to the code to support this in a > >> very flexible way. I think this could be much easier if we confine > >> ourselves to two regions (one contiguous lower (< 4 GB) and one > >> contiguous upper region (>4 GB)), so we don't need to support arbitrary > >> per VCPU addresses, but could just use the 1st or 2nd map depending on > >> the VCPU number. > >> Is this too hackish? > >> If not, I would add another vgic_addr type (like > >> KVM_VGIC_V3_ADDR_TYPE_REDIST_UPPER or so) to be used from userland and > >> use that in the handle_mmio region detection. > >> Let me know if that sounds reasonable. > >> > > The point that I've been trying to make sure we think about is if we'll > > regret not being able to fragment the redistributor regions a bit. Even > > if it's technically possible, we may regret requiring a huge contigous > > allocation in the guest physical address space. But maybe we don't care > > when we have 40 bits to play with? > > 40 bits are more than enough. But are we OK with using only memory above > 4GB? Is there some code before the Linux kernel that is limited to 4GB? > I am thinking about 32bit guests in particular, which may have some > firmware blob executed before which may not use the MMU. > > If this is not an issue, I'd rather stay with one contiguous region - at > least for the itme being. The current GICv3 code has a limit of 255 > VCPUs anyway, so this requires at most 32MB, which should be easily > fitted anywhere. > > Should we later need to extend the number of VCPUs, we can in the worst > case adjust the code to support split regions if the 4GB limit issue > persists. This would be done via a new KVM capability and some new > register groups in the KVM device ioctl to set a second (or following) > region, so in a backwards compatible way. > ok, sounds reasonable. I'll shut up then. Thanks, -Christoffer _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
