Re: [PATCH] KVM: s390: pci: Hook to access KVM lowlevel from VFIO

[Matthew Rosato]

On 8/18/22 6:23 AM, Pierre Morel wrote:
> We have a cross dependency between KVM and VFIO.

maybe add something like 'when using s390 vfio_pci_zdev extensions for PCI passthrough'

> To be able to keep both subsystem modular we add a registering
> hook inside the S390 core code.
> 
> This fixes a build problem when VFIO is built-in and KVM is built
> as a module or excluded.

s/or excluded//

There's no problem when KVM is excluded, that forces CONFIG_VFIO_PCI_ZDEV_KVM=n because of the 'depends on S390 && KVM'.

> 
> Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> Fixes: 09340b2fca007 ("KVM: s390: pci: add routines to start/stop inter..")
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  arch/s390/include/asm/kvm_host.h | 17 ++++++-----------
>  arch/s390/kvm/pci.c              | 10 ++++++----
>  arch/s390/pci/Makefile           |  2 ++
>  arch/s390/pci/pci_kvm_hook.c     | 11 +++++++++++
>  drivers/vfio/pci/vfio_pci_zdev.c |  8 ++++++--
>  5 files changed, 31 insertions(+), 17 deletions(-)
>  create mode 100644 arch/s390/pci/pci_kvm_hook.c
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index f39092e0ceaa..8312ed9d1937 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -1038,16 +1038,11 @@ static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}
>  #define __KVM_HAVE_ARCH_VM_FREE
>  void kvm_arch_free_vm(struct kvm *kvm);
>  
> -#ifdef CONFIG_VFIO_PCI_ZDEV_KVM
> -int kvm_s390_pci_register_kvm(struct zpci_dev *zdev, struct kvm *kvm);
> -void kvm_s390_pci_unregister_kvm(struct zpci_dev *zdev);
> -#else
> -static inline int kvm_s390_pci_register_kvm(struct zpci_dev *dev,
> -					    struct kvm *kvm)
> -{
> -	return -EPERM;
> -}
> -static inline void kvm_s390_pci_unregister_kvm(struct zpci_dev *dev) {}
> -#endif
> +struct kvm_register_hook {

Nit: zpci_kvm_register_hook ?  Just to make it clear it's for zpci.

> +	int (*kvm_register)(void *opaque, struct kvm *kvm);
> +	void (*kvm_unregister)(void *opaque);
> +};
> +
> +extern struct kvm_register_hook kvm_pci_hook;

Nit: kvm_zpci_hook ?

>  
>  #endif
> diff --git a/arch/s390/kvm/pci.c b/arch/s390/kvm/pci.c
> index 4946fb7757d6..e173fce64c4f 100644
> --- a/arch/s390/kvm/pci.c
> +++ b/arch/s390/kvm/pci.c
> @@ -431,8 +431,9 @@ static void kvm_s390_pci_dev_release(struct zpci_dev *zdev)
>   * available, enable them and let userspace indicate whether or not they will
>   * be used (specify SHM bit to disable).
>   */
> -int kvm_s390_pci_register_kvm(struct zpci_dev *zdev, struct kvm *kvm)
> +static int kvm_s390_pci_register_kvm(void *opaque, struct kvm *kvm)
>  {
> +	struct zpci_dev *zdev = opaque;
>  	int rc;
>  
>  	if (!zdev)
> @@ -510,10 +511,10 @@ int kvm_s390_pci_register_kvm(struct zpci_dev *zdev, struct kvm *kvm)
>  	kvm_put_kvm(kvm);
>  	return rc;
>  }
> -EXPORT_SYMBOL_GPL(kvm_s390_pci_register_kvm);
>  
> -void kvm_s390_pci_unregister_kvm(struct zpci_dev *zdev)
> +static void kvm_s390_pci_unregister_kvm(void *opaque)
>  {
> +	struct zpci_dev *zdev = opaque;
>  	struct kvm *kvm;
>  
>  	if (!zdev)
> @@ -566,7 +567,6 @@ void kvm_s390_pci_unregister_kvm(struct zpci_dev *zdev)
>  
>  	kvm_put_kvm(kvm);
>  }
> -EXPORT_SYMBOL_GPL(kvm_s390_pci_unregister_kvm);
>  
>  void kvm_s390_pci_init_list(struct kvm *kvm)
>  {
> @@ -678,6 +678,8 @@ int kvm_s390_pci_init(void)
>  
>  	spin_lock_init(&aift->gait_lock);
>  	mutex_init(&aift->aift_lock);
> +	kvm_pci_hook.kvm_register = kvm_s390_pci_register_kvm;
> +	kvm_pci_hook.kvm_unregister = kvm_s390_pci_unregister_kvm;
>  
>  	return 0;
>  }
> diff --git a/arch/s390/pci/Makefile b/arch/s390/pci/Makefile
> index bf557a1b789c..c02dbfb415d9 100644
> --- a/arch/s390/pci/Makefile
> +++ b/arch/s390/pci/Makefile
> @@ -7,3 +7,5 @@ obj-$(CONFIG_PCI)	+= pci.o pci_irq.o pci_dma.o pci_clp.o pci_sysfs.o \
>  			   pci_event.o pci_debug.o pci_insn.o pci_mmio.o \
>  			   pci_bus.o
>  obj-$(CONFIG_PCI_IOV)	+= pci_iov.o
> +
> +obj-y += pci_kvm_hook.o

I guess it doesn't harm anything to add this unconditionally, but I think it would also be OK to just include this in the CONFIG_PCI list - vfio_pci_zdev and arch/s390/kvm/pci all rely on CONFIG_PCI via CONFIG_VFIO_PCI_ZDEV_KVM which implies PCI via VFIO_PCI.

> diff --git a/arch/s390/pci/pci_kvm_hook.c b/arch/s390/pci/pci_kvm_hook.c
> new file mode 100644
> index 000000000000..9d8799b72dbf
> --- /dev/null
> +++ b/arch/s390/pci/pci_kvm_hook.c
> @@ -0,0 +1,11 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * VFIO ZPCI devices support
> + *
> + * Copyright (C) IBM Corp. 2022.  All rights reserved.
> + *	Author(s): Pierre Morel <pmorel@xxxxxxxxxxxxx>
> + */
> +#include <linux/kvm_host.h>
> +
> +struct kvm_register_hook kvm_pci_hook;
> +EXPORT_SYMBOL_GPL(kvm_pci_hook);

Following the comments above, zpci_kvm_register_hook, kvm_zpci_hook ?

I'm not sure if this really needs to be in a separate file or if it could just go into arch/s390/pci.c with the zpci_aipb -- If going the route of a separate file, up to Niklas whether he wants this under the S390 PCI maintainership or added to the list for s390 vfio-pci like arch/kvm/pci* and vfio_pci_zdev.

> diff --git a/drivers/vfio/pci/vfio_pci_zdev.c b/drivers/vfio/pci/vfio_pci_zdev.c
> index e163aa9f6144..3b7a707e2fe5 100644
> --- a/drivers/vfio/pci/vfio_pci_zdev.c
> +++ b/drivers/vfio/pci/vfio_pci_zdev.c
> @@ -151,7 +151,10 @@ int vfio_pci_zdev_open_device(struct vfio_pci_core_device *vdev)
>  	if (!vdev->vdev.kvm)
>  		return 0;
>  
> -	return kvm_s390_pci_register_kvm(zdev, vdev->vdev.kvm);
> +	if (kvm_pci_hook.kvm_register)
> +		return kvm_pci_hook.kvm_register(zdev, vdev->vdev.kvm);
> +
> +	return -ENOENT;
>  }
>  
>  void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev)
> @@ -161,5 +164,6 @@ void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev)
>  	if (!zdev || !vdev->vdev.kvm)
>  		return;
>  
> -	kvm_s390_pci_unregister_kvm(zdev);
> +	if (kvm_pci_hook.kvm_unregister)
> +		return kvm_pci_hook.kvm_unregister(zdev);

No need for the return here, this is a void function calling a void function.


Overall, this looks good to me and survives a series of compile and device passthrough tests on my end, just a matter of a few of these minor comments above.  Thanks for tackling this Pierre!