[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [XEN PATCH 1/2] hvm: Support more than 32 VCPUS when migrating.
On 09/04/14 10:33, Ian Campbell wrote: > On Tue, 2014-04-08 at 14:53 -0400, Konrad Rzeszutek Wilk wrote: >> On Tue, Apr 08, 2014 at 08:18:48PM +0200, Roger Pau Monnà wrote: >>> On 08/04/14 19:25, konrad@xxxxxxxxxx wrote: >>>> From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> >>>> >>>> When we migrate an HVM guest, by default our shared_info can >>>> only hold up to 32 CPUs. As such the hypercall >>>> VCPUOP_register_vcpu_info was introduced which allowed us to >>>> setup per-page areas for VCPUs. This means we can boot PVHVM >>>> guest with more than 32 VCPUs. During migration the per-cpu >>>> structure is allocated fresh by the hypervisor (vcpu_info_mfn >>>> is set to INVALID_MFN) so that the newly migrated guest >>>> can do make the VCPUOP_register_vcpu_info hypercall. >>>> >>>> Unfortunatly we end up triggering this condition: >>>> /* Run this command on yourself or on other offline VCPUS. */ >>>> if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) ) >>>> >>>> which means we are unable to setup the per-cpu VCPU structures >>>> for running vCPUS. The Linux PV code paths make this work by >>>> iterating over every vCPU with: >>>> >>>> 1) is target CPU up (VCPUOP_is_up hypercall?) >>>> 2) if yes, then VCPUOP_down to pause it. >>>> 3) VCPUOP_register_vcpu_info >>>> 4) if it was down, then VCPUOP_up to bring it back up >>>> >>>> But since VCPUOP_down, VCPUOP_is_up, and VCPUOP_up are >>>> not allowed on HVM guests we can't do this. This patch >>>> enables this. >>> >>> Hmmm, this looks like a very convoluted approach to something that could >>> be solved more easily IMHO. What we do on FreeBSD is put all vCPUs into >>> suspension, which means that all vCPUs except vCPU#0 will be in the >>> cpususpend_handler, see: >>> >>> http://svnweb.freebsd.org/base/head/sys/amd64/amd64/mp_machdep.c?revision=263878&view=markup#l1460 >> >> How do you 'suspend' them? If I remember there is a disadvantage of doing >> this as you have to bring all the CPUs "offline". That in Linux means using >> the stop_machine which is pretty big hammer and increases the latency for >> migration. > > Yes, this is why the ability to have the toolstack save/restore the > secondary vcpu state was added. It's especially important for > checkpointing, but it's relevant to regular migrate as a performance > improvement too. > > It's not just stop-machine, IIRC it's a tonne of udev events relating to > cpus off/onlinign etc too and all the userspace activity which that > implies. Well, what it's done on FreeBSD is nothing like that, it's called the cpususpend handler, but it's not off-lining CPUs or anything like that, it just places the CPU in a while loop inside of an IPI handler, so we can do something like this will all APs: while (suspended) pause(); register_vcpu_info(); So the registration of the vcpu_info area happens just after the CPU is waken from suspension and before it leaves the IPI handler, and it's the CPU itself the one that calls VCPUOP_register_vcpu_info (so we can avoid the gate in Xen that prevents registering the vcpu_info area for CPUs different that ourself). Roger. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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