On Sat, Mar 21, 2015 at 8:28 PM, Alexander Graf <agraf@xxxxxxx> wrote: > > > On 20.03.15 16:51, Bharata B Rao wrote: >> On Fri, Mar 20, 2015 at 12:34:18PM +0100, Alexander Graf wrote: >>> >>> >>> On 20.03.15 12:26, Paul Mackerras wrote: >>>> On Fri, Mar 20, 2015 at 12:01:32PM +0100, Alexander Graf wrote: >>>>> >>>>> >>>>> On 20.03.15 10:39, Paul Mackerras wrote: >>>>>> From: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx> >>>>>> >>>>>> Since KVM isn't equipped to handle closure of vcpu fd from userspace(QEMU) >>>>>> correctly, certain work arounds have to be employed to allow reuse of >>>>>> vcpu array slot in KVM during cpu hot plug/unplug from guest. One such >>>>>> proposed workaround is to park the vcpu fd in userspace during cpu unplug >>>>>> and reuse it later during next hotplug. >>>>>> >>>>>> More details can be found here: >>>>>> KVM: https://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg102839.html >>>>>> QEMU: http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg00859.html >>>>>> >>>>>> In order to support this workaround with PowerPC KVM, don't create or >>>>>> initialize ICP if the vCPU is found to be already associated with an ICP. >>>>>> >>>>>> Signed-off-by: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx> >>>>>> Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx> >>>>> >>>>> This probably makes some sense, but please make sure that user space has >>>>> some way to figure out whether hotplug works at all. >>>> >>>> Bharata is working on the qemu side of all this, so I assume he has >>>> that covered. >>> >>> Well, so far the kernel doesn't expose anything he can query, so I >>> suppose he just blindly assumes that older host kernels will randomly >>> break and nobody cares. I'd rather prefer to see a CAP exposed that qemu >>> can check on. >> >> I see that you have already taken this into your tree. I have an updated >> patch to expose a CAP. If the below patch looks ok, then let me know how >> you would prefer to take this patch in. >> >> Regards, >> Bharata. >> >> KVM: PPC: BOOK3S: Allow reuse of vCPU object >> >> From: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx> >> >> Since KVM isn't equipped to handle closure of vcpu fd from userspace(QEMU) >> correctly, certain work arounds have to be employed to allow reuse of >> vcpu array slot in KVM during cpu hot plug/unplug from guest. One such >> proposed workaround is to park the vcpu fd in userspace during cpu unplug >> and reuse it later during next hotplug. >> >> More details can be found here: >> KVM: https://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg102839.html >> QEMU: http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg00859.html >> >> In order to support this workaround with PowerPC KVM, don't create or >> initialize ICP if the vCPU is found to be already associated with an ICP. >> User space (QEMU) can reuse the vCPU after checking for the availability >> of KVM_CAP_SPAPR_REUSE_VCPU capability. >> >> Signed-off-by: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx> >> --- >> arch/powerpc/kvm/book3s_xics.c | 9 +++++++-- >> arch/powerpc/kvm/powerpc.c | 12 ++++++++++++ >> include/uapi/linux/kvm.h | 1 + >> 3 files changed, 20 insertions(+), 2 deletions(-) >> >> diff --git a/arch/powerpc/kvm/book3s_xics.c b/arch/powerpc/kvm/book3s_xics.c >> index a4a8d9f..ead3a35 100644 >> --- a/arch/powerpc/kvm/book3s_xics.c >> +++ b/arch/powerpc/kvm/book3s_xics.c >> @@ -1313,8 +1313,13 @@ int kvmppc_xics_connect_vcpu(struct kvm_device *dev, struct kvm_vcpu *vcpu, >> return -EPERM; >> if (xics->kvm != vcpu->kvm) >> return -EPERM; >> - if (vcpu->arch.irq_type) >> - return -EBUSY; >> + >> + /* >> + * If irq_type is already set, don't reinialize but >> + * return success allowing this vcpu to be reused. >> + */ >> + if (vcpu->arch.irq_type != KVMPPC_IRQ_DEFAULT) >> + return 0; >> >> r = kvmppc_xics_create_icp(vcpu, xcpu); >> if (!r) >> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c >> index 27c0fac..5b7007c 100644 >> --- a/arch/powerpc/kvm/powerpc.c >> +++ b/arch/powerpc/kvm/powerpc.c >> @@ -564,6 +564,18 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) >> r = 1; >> break; >> #endif >> + case KVM_CAP_SPAPR_REUSE_VCPU: >> + /* >> + * Kernel currently doesn't support closing of vCPU fd from >> + * user space (QEMU) correctly. Hence the option available >> + * is to park the vCPU fd in user space whenever a guest >> + * CPU is hot removed and reuse the same later when another >> + * guest CPU is hotplugged. This capability determines whether >> + * it is safe to assume if parking of vCPU fd and reuse from >> + * user space works for sPAPR guests. > > I don't see how the code you're changing here has anything to do with > parking vcpus. It's all about being able to call connect on an already > connected vcpu and not erroring out. Please reflect this in the cap name > and description. > > You also need to update Documentation/virtual/kvm/api.txt. > > Furthermore, thinking about this a bit more, I might still miss the > exact case why you need this. Why is QEMU issuing a connect again? Could > it maybe just not do it? Thinking more, I see that I can handle this entirely in user space. So no need for this patch. Sorry for the noise :( Regards, Bharata. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
