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? -Christoffer _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
