Re: [RFC PATCH 3/3] kvm: Add VFIO device for handling IOMMU cache coherency

Michael S. Tsirkin
2013-09-13T12:37:53Z

On Thu, Sep 12, 2013 at 03:23:15PM -0600, Alex Williamson wrote:
> So far we've succeeded at making KVM and VFIO mostly unaware of each
> other, but there's any important point where that breaks down. Intel
> VT-d hardware may or may not support snoop control. When snoop
> control is available, intel-iommu promotes No-Snoop transactions on
> PCIe to be cache coherent. That allows KVM to handle things like the
> x86 WBINVD opcode as a nop. When the hardware does not support this,
> KVM must implement a hardware visible WBINVD for the guest.
>
> We could simply let userspace tell KVM how to handle WBINVD, but it's
> privileged for a reason. Allowing an arbitrary user to enable
> physical WBINVD gives them a more access to the hardware. Previously,
> this has only been enabled for guests supporting legacy PCI device
> assignment. In such cases it's necessary for proper guest execution.
> We therefore create a new KVM-VFIO virtual device. The user can add
> and remove VFIO groups to this device via file descriptors. KVM
> makes use of the VFIO external user interface to validate that the
> user has access to physical hardware and gets the coherency state of
> the IOMMU from VFIO. This provides equivalent functionality to
> legacy KVM assignment, while keeping (nearly) all the bits isolated.

So how is the isolation handled then?
How is this better than a ioctl to grant WBINVD to guest?
kvm char device can be opened by any user,
so any user can grant itself these priveledges.
What did I miss?

> The one intrusion is the resulting flag indicating the coherency
> state. For this RFC it's placed on the x86 kvm_arch struct, however
> I know POWER has interest in using the VFIO external user interface,
> and I'm hoping we can share a common KVM-VFIO device. Perhaps they
> care about No-Snoop handling as well or the code can be #ifdef'd.
>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> Documentation/virtual/kvm/devices/vfio.txt | 22 +++
> arch/x86/include/asm/kvm_host.h | 1
> arch/x86/kvm/Makefile | 2
> arch/x86/kvm/vmx.c | 5 -
> arch/x86/kvm/x86.c | 5 -
> include/linux/kvm_host.h | 1
> include/uapi/linux/kvm.h | 4
> virt/kvm/kvm_main.c | 3
> virt/kvm/vfio.c | 237 ++++++++++++++++++++++++++++
> 9 files changed, 275 insertions(+), 5 deletions(-)
> create mode 100644 Documentation/virtual/kvm/devices/vfio.txt
> create mode 100644 virt/kvm/vfio.c
>
> diff --git a/Documentation/virtual/kvm/devices/vfio.txt b/Documentation/virtual/kvm/devices/vfio.txt
> new file mode 100644
> index 0000000..831e6a6
> --- /dev/null
> +++ b/Documentation/virtual/kvm/devices/vfio.txt
> @@ -0,0 +1,22 @@
> +VFIO virtual device
> +===================
> +
> +Device types supported:
> + KVM_DEV_TYPE_VFIO
> +
> +Only one VFIO instance may be created per VM. The created device
> +tracks VFIO groups in use by the VM and features of those groups
> +important to the correctness and acceleration of the VM. As groups
> +are enabled and disabled for use by the VM, KVM should be updated
> +about their presence. When registered with KVM, a reference to the
> +VFIO-group is held by KVM.
> +
> +Groups:
> + KVM_DEV_VFIO_ADD_GROUP
> + KVM_DEV_VFIO_DEL_GROUP
> +
> +Each takes a int32_t file descriptor for kvm_device_attr.addr and
> +does not support any group device kvm_device_attr.attr.
> +
> +RFC - Should we use Group KVM_DEV_VFIO_GROUP with Attributes
> + KVM_DEV_VFIO_GROUP_ADD & DEL?
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c76ff74..5b9350d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -588,6 +588,7 @@ struct kvm_arch {
>
> spinlock_t pvclock_gtod_sync_lock;
> bool use_master_clock;
> + bool vfio_noncoherent;
> u64 master_kernel_ns;
> cycle_t master_cycle_now;
>
> diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
> index bf4fb04..25d22b2 100644
> --- a/arch/x86/kvm/Makefile
> +++ b/arch/x86/kvm/Makefile
> @@ -9,7 +9,7 @@ KVM := ../../../virt/kvm
>
> kvm-y += $(KVM)/kvm_main.o $(KVM)/ioapic.o \
> $(KVM)/coalesced_mmio.o $(KVM)/irq_comm.o \
> - $(KVM)/eventfd.o $(KVM)/irqchip.o
> + $(KVM)/eventfd.o $(KVM)/irqchip.o $(KVM)/vfio.o
> kvm-$(CONFIG_KVM_DEVICE_ASSIGNMENT) += $(KVM)/assigned-dev.o $(KVM)/iommu.o
> kvm-$(CONFIG_KVM_ASYNC_PF) += $(KVM)/async_pf.o
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 1f1da43..94f7786 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -7395,8 +7395,9 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> */
> if (is_mmio)
> ret = MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;
> - else if (vcpu->kvm->arch.iommu_domain &&
> - !(vcpu->kvm->arch.iommu_flags & KVM_IOMMU_CACHE_COHERENCY))
> + else if (vcpu->kvm->arch.vfio_noncoherent ||
> + vcpu->kvm->arch.iommu_domain &&
> + !(vcpu->kvm->arch.iommu_flags & KVM_IOMMU_CACHE_COHERENCY))
> ret = kvm_get_guest_memory_type(vcpu, gfn) <<
> VMX_EPT_MT_EPTE_SHIFT;
> else
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e5ca72a..406ba6f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2715,8 +2715,9 @@ static void wbinvd_ipi(void *garbage)
>
> static bool need_emulate_wbinvd(struct kvm_vcpu *vcpu)
> {
> - return vcpu->kvm->arch.iommu_domain &&
> - !(vcpu->kvm->arch.iommu_flags & KVM_IOMMU_CACHE_COHERENCY);
> + return vcpu->kvm->arch.vfio_noncoherent ||
> + (vcpu->kvm->arch.iommu_domain &&
> + !(vcpu->kvm->arch.iommu_flags & KVM_IOMMU_CACHE_COHERENCY));
> }
>
> void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ca645a0..615f0c3 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1065,6 +1065,7 @@ struct kvm_device *kvm_device_from_filp(struct file *filp);
>
> extern struct kvm_device_ops kvm_mpic_ops;
> extern struct kvm_device_ops kvm_xics_ops;
> +extern struct kvm_device_ops kvm_vfio_ops;
>
> #ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT
>
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 99c2533..8869616 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -843,6 +843,10 @@ struct kvm_device_attr {
> #define KVM_DEV_TYPE_FSL_MPIC_20 1
> #define KVM_DEV_TYPE_FSL_MPIC_42 2
> #define KVM_DEV_TYPE_XICS 3
> +#define KVM_DEV_TYPE_VFIO 4
> +
> +#define KVM_DEV_VFIO_ADD_GROUP 1
> +#define KVM_DEV_VFIO_DEL_GROUP 2
>
> /*
> * ioctls for VM fds
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d9cad4d..1a20425 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2269,6 +2269,9 @@ static int kvm_ioctl_create_device(struct kvm *kvm,
> ops = &kvm_xics_ops;
> break;
> #endif
> + case KVM_DEV_TYPE_VFIO:
> + ops = &kvm_vfio_ops;
> + break;
> default:
> return -ENODEV;
> }
> diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> new file mode 100644
> index 0000000..9a2faff
> --- /dev/null
> +++ b/virt/kvm/vfio.c
> @@ -0,0 +1,237 @@
> +/*
> + * VFIO bridge
> + *
> + * Copyright (C) 2013 Red Hat, Inc. All rights reserved.
> + * Author: Alex Williamson <alex.williamson@redhat.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +
> +#include <linux/errno.h>
> +#include <linux/file.h>
> +#include <linux/kvm_host.h>
> +#include <linux/list.h>
> +#include <linux/module.h>
> +#include <linux/mutex.h>
> +#include <linux/slab.h>
> +#include <linux/vfio.h>
> +
> +struct kvm_vfio_group {
> + struct list_head node;
> + struct vfio_group *vfio_group;
> +};
> +
> +struct kvm_vfio {
> + struct list_head group_list;
> + struct mutex lock;
> +};
> +
> +static struct vfio_group *kvm_vfio_group_get_external_user(struct file *filep)
> +{
> + struct vfio_group *vfio_group;
> + struct vfio_group *(*fn)(struct file *);
> +
> + fn = symbol_get(vfio_group_get_external_user);
> + if (!fn)
> + return ERR_PTR(-EINVAL);
> +
> + vfio_group = fn(filep);
> +
> + symbol_put(vfio_group_get_external_user);
> +
> + return vfio_group;
> +}
> +
> +static void kvm_vfio_group_put_external_user(struct vfio_group *vfio_group)
> +{
> + void (*fn)(struct vfio_group *);
> +
> + fn = symbol_get(vfio_group_put_external_user);
> + if (!fn)
> + return;
> +
> + fn(vfio_group);
> +
> + symbol_put(vfio_group_put_external_user);
> +}
> +
> +static int kvm_vfio_external_user_check_extension(struct vfio_group *vfio_group,
> + unsigned long arg)
> +{
> + int (*fn)(struct vfio_group *, unsigned long);
> + int ret;
> +
> + fn = symbol_get(vfio_external_user_check_extension);
> + if (!fn)
> + return -EINVAL;
> +
> + ret = fn(vfio_group, arg);
> +
> + symbol_put(vfio_group_put_external_user);
> +
> + return ret;
> +}
> +
> +static void kvm_vfio_update_iommu_coherency(struct kvm_device *dev)
> +{
> + struct kvm_vfio *kv = dev->private;
> + bool coherent = true;
> + struct kvm_vfio_group *kvg;
> +
> + mutex_lock(&kv->lock);
> +
> + list_for_each_entry(kvg, &kv->group_list, node) {
> + if (!kvm_vfio_external_user_check_extension(kvg->vfio_group,
> + VFIO_IOMMU_CAP_CACHE_COHERENCY)) {
> + coherent = false;
> + break;
> + }
> + }
> +
> + mutex_unlock(&kv->lock);
> +
> + dev->kvm->arch.vfio_noncoherent = !coherent;
> +}
> +
> +static int kvm_vfio_set_attr(struct kvm_device *dev,
> + struct kvm_device_attr *attr)
> +{
> + struct kvm_vfio *kv = dev->private;
> + struct fd f;
> + struct vfio_group *vfio_group;
> + struct kvm_vfio_group *kvg;
> + int ret;
> +
> + switch (attr->group) {
> + case KVM_DEV_VFIO_ADD_GROUP:
> + f = fdget(attr->addr);
> + if (!f.file)
> + return -EBADF;
> +
> + vfio_group = kvm_vfio_group_get_external_user(f.file);
> + fdput(f);
> +
> + if (IS_ERR(vfio_group))
> + return PTR_ERR(vfio_group);
> +
> + mutex_lock(&kv->lock);
> +
> + list_for_each_entry(kvg, &kv->group_list, node) {
> + if (kvg->vfio_group == vfio_group) {
> + mutex_unlock(&kv->lock);
> + kvm_vfio_group_put_external_user(vfio_group);
> + return -EEXIST;
> + }
> + }
> +
> + kvg = kzalloc(sizeof(*kvg), GFP_KERNEL);
> + if (!kvg) {
> + mutex_unlock(&kv->lock);
> + kvm_vfio_group_put_external_user(vfio_group);
> + return -ENOMEM;
> + }
> +
> + list_add_tail(&kvg->node, &kv->group_list);
> + kvg->vfio_group = vfio_group;
> +
> + mutex_unlock(&kv->lock);
> +
> + kvm_vfio_update_iommu_coherency(dev);
> +
> + return 0;
> +
> + case KVM_DEV_VFIO_DEL_GROUP:
> + f = fdget(attr->addr);
> + if (!f.file)
> + return -EBADF;
> +
> + vfio_group = kvm_vfio_group_get_external_user(f.file);
> + fdput(f);
> +
> + if (IS_ERR(vfio_group))
> + return PTR_ERR(vfio_group);
> +
> + ret = -ENOENT;
> +
> + mutex_lock(&kv->lock);
> +
> + list_for_each_entry(kvg, &kv->group_list, node) {
> + if (kvg->vfio_group != vfio_group)
> + continue;
> +
> + list_del(&kvg->node);
> + kvm_vfio_group_put_external_user(kvg->vfio_group);
> + kfree(kvg);
> + ret = 0;
> + break;
> + }
> +
> + mutex_unlock(&kv->lock);
> + kvm_vfio_group_put_external_user(vfio_group);
> +
> + kvm_vfio_update_iommu_coherency(dev);
> +
> + return ret;
> + }
> +
> + return -ENXIO;
> +}
> +
> +static int kvm_vfio_has_attr(struct kvm_device *dev,
> + struct kvm_device_attr *attr)
> +{
> + switch (attr->group) {
> + case KVM_DEV_VFIO_ADD_GROUP:
> + case KVM_DEV_VFIO_DEL_GROUP:
> + return 0;
> + }
> +
> + return -ENXIO;
> +}
> +
> +static void kvm_vfio_destroy(struct kvm_device *dev)
> +{
> + struct kvm_vfio *kv = dev->private;
> + struct kvm_vfio_group *kvg, *tmp;
> +
> + list_for_each_entry_safe(kvg, tmp, &kv->group_list, node) {
> + kvm_vfio_group_put_external_user(kvg->vfio_group);
> + list_del(&kvg->node);
> + kfree(kvg);
> + }
> +
> + dev->kvm->arch.vfio_noncoherent = false;
> + kfree(kv);
> +}
> +
> +static int kvm_vfio_create(struct kvm_device *dev, u32 type)
> +{
> + struct kvm_device *tmp;
> + struct kvm_vfio *kv;
> +
> + /* Only one VFIO "device" per VM */
> + list_for_each_entry(tmp, &dev->kvm->devices, vm_node)
> + if (tmp->ops == &kvm_vfio_ops)
> + return -EBUSY;
> +
> + kv = kzalloc(sizeof(*kv), GFP_KERNEL);
> + if (!kv)
> + return -ENOMEM;
> +
> + INIT_LIST_HEAD(&kv->group_list);
> + mutex_init(&kv->lock);
> +
> + dev->private = kv;
> +
> + return 0;
> +}
> +
> +struct kvm_device_ops kvm_vfio_ops = {
> + .name = "kvm-vfio",
> + .create = kvm_vfio_create,
> + .destroy = kvm_vfio_destroy,
> + .set_attr = kvm_vfio_set_attr,
> + .has_attr = kvm_vfio_has_attr,
> +};