Re: [PATCH 16/17] KVM: PPC: MPIC: Add support for KVM_IRQ_LINE

Alexander Graf
2013-04-25T21:13:40Z

On 25.04.2013, at 21:03, Scott Wood wrote:

> On 04/25/2013 09:49:23 AM, Alexander Graf wrote:
>> On 25.04.2013, at 13:30, Alexander Graf wrote:
>> >
>> > On 19.04.2013, at 20:51, Scott Wood wrote:
>> >
>> >> On 04/19/2013 09:06:27 AM, Alexander Graf wrote:
>> >>> Now that all pieces are in place for reusing generic irq infrastructure,
>> >>> we can copy x86's implementation of KVM_IRQ_LINE irq injection and simply
>> >>> reuse it for PPC, as it will work there just as well.
>> >>> Signed-off-by: Alexander Graf <agraf@suse.de>
>> >>> ---
>> >>> arch/powerpc/include/uapi/asm/kvm.h | 1 +
>> >>> arch/powerpc/kvm/powerpc.c | 13 +++++++++++++
>> >>> 2 files changed, 14 insertions(+), 0 deletions(-)
>> >>> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
>> >>> index 3537bf3..dbb2ac2 100644
>> >>> --- a/arch/powerpc/include/uapi/asm/kvm.h
>> >>> +++ b/arch/powerpc/include/uapi/asm/kvm.h
>> >>> @@ -26,6 +26,7 @@
>> >>> #define __KVM_HAVE_SPAPR_TCE
>> >>> #define __KVM_HAVE_PPC_SMT
>> >>> #define __KVM_HAVE_IRQCHIP
>> >>> +#define __KVM_HAVE_IRQ_LINE
>> >>> struct kvm_regs {
>> >>> __u64 pc;
>> >>> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
>> >>> index c431fea..874c106 100644
>> >>> --- a/arch/powerpc/kvm/powerpc.c
>> >>> +++ b/arch/powerpc/kvm/powerpc.c
>> >>> @@ -33,6 +33,7 @@
>> >>> #include <asm/cputhreads.h>
>> >>> #include <asm/irqflags.h>
>> >>> #include "timing.h"
>> >>> +#include "irq.h"
>> >>> #include "../mm/mmu_decl.h"
>> >>> #define CREATE_TRACE_POINTS
>> >>> @@ -945,6 +946,18 @@ static int kvm_vm_ioctl_get_pvinfo(struct kvm_ppc_pvinfo *pvinfo)
>> >>> return 0;
>> >>> }
>> >>> +int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_event,
>> >>> + bool line_status)
>> >>> +{
>> >>> + if (!irqchip_in_kernel(kvm))
>> >>> + return -ENXIO;
>> >>> +
>> >>> + irq_event->status = kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID,
>> >>> + irq_event->irq, irq_event->level,
>> >>> + line_status);
>> >>> + return 0;
>> >>> +}
>> >>
>> >> As Paul noted in the XICS patchset, this could reference an MPIC that has gone away if the user never attached any vcpus and then closed the MPIC fd. It's not a reasonable use case, but it could be used malicously to get the kernel to access a bad pointer. The irqchip_in_kernel check helps somewhat, but it's meant for ensuring that the creation has happened -- it's racy if used for ensuring that destruction hasn't happened.
>> >>
>> >> The problem is rooted in the awkwardness of performing an operation that logically should be on the MPIC fd, but is instead being done on the vm fd.
>> >>
>> >> I think these three steps would fix it (the first two seem like things we should be doing anyway):
>> >> - During MPIC destruction, make sure MPIC deregisters all routes that reference it.
>> >> - In kvm_set_irq(), do not release the RCU read lock until after the set() function has been called.
>> >> - Do not hook up kvm_send_userspace_msi() to MPIC or other new irqchips, as that bypasses the RCU lock. It could be supported as a device fd ioctl if desired, or it could be reworked to operate on an RCU-managed list of MSI handlers, though MPIC really doesn't need this at all.
>> >
>> > Can't we just add an RCU lock in the send_userspace_msi case? I don't think we should handle MSIs any differently from normal IRQs.
>
> Well, you can't *just* add the RCU lock -- you need to add data to be managed via RCU (e.g. a list of MSI callbacks, or at least a boolean indicating whether calling the MSI code is OK).

Well, we'd just access a random pin routing :).

>
>> In fact I'm having a hard time verifying that we're always accessing things with proper locks held. I'm pretty sure we're missing a few cases.
>
> Any path in particular?

I'm already getting confused on whether normal MMIO accesses are always safe.

>
>> So how about we delay mpic destruction to vm destruction? We simply add one user too many when we spawn the mpic and put it on vm_destruct. That way users _can_ destroy mpics, but they will only be really free'd once the vm is also gone.
>
> That's what we originally had before the fd conversion. If we want it again, we'll need to go back to maintaining a list of devices in KVM (though it could be a linked list now that we don't need to use it for lookups), or have some hardcoded MPIC hack.

Well, we could have an anonymous linked list of device pointers with a simple registration function. That way it's generic enough for any device to be kept alive until vm destruction if it wants that.

> IIRC I said back then that converting to fd would make destruction ordering more of a pain...

I usually like to pick the raisins from everything I can. So while I like the fd approach for its universally understandable scheme, simplicity of use, extensibility of ioctls etc, I don't really like the headaches that come with destroying a device while a VM is running. So having a device keep itself alive until the VM is gone is the best of all worlds :).

Alex