Re: [PATCH v3 4/5] vhost-user-dev: Add cache BAR

[Albert Esteve <aesteve@redhat.com>]

On Mon, Nov 25, 2024 at 5:16 PM Albert Esteve <aesteve@redhat.com> wrote:
>
> On Tue, Sep 17, 2024 at 10:27 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Thu, Sep 12, 2024 at 04:53:34PM +0200, Albert Esteve wrote:
> > > Add a cache BAR in the vhost-user-device
> > > into which files can be directly mapped.
> > >
> > > The number, shmid, and size of the VIRTIO Shared
> > > Memory subregions is retrieved through a get_shmem_config
> > > message sent by the vhost-user-base module
> > > on the realize step, after virtio_init().
> > >
> > > By default, if VHOST_USER_PROTOCOL_F_SHMEM
> > > feature is not supported by the backend,
> > > there is no cache.
> > >
> > > Signed-off-by: Albert Esteve <aesteve@redhat.com>
> > > ---
> >
> > Not all devices derive from vhost-user-base.c so this does not offer
> > full coverage. I think that's okay since few devices currently use
> > VIRTIO Shared Memory Regions. A note about this in the commit
> > description would be useful though. Which vhost-user devices gain VIRTIO
> > Shared Memory Region support and what should you do if your device is
> > not included in this list?
> >
> > >  hw/virtio/vhost-user-base.c       | 37 +++++++++++++++++++++++++++--
> > >  hw/virtio/vhost-user-device-pci.c | 39 ++++++++++++++++++++++++++++---
> > >  2 files changed, 71 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/hw/virtio/vhost-user-base.c b/hw/virtio/vhost-user-base.c
> > > index 2bc3423326..f2597d021a 100644
> > > --- a/hw/virtio/vhost-user-base.c
> > > +++ b/hw/virtio/vhost-user-base.c
> > > @@ -271,7 +271,9 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
> > >  {
> > >      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > >      VHostUserBase *vub = VHOST_USER_BASE(dev);
> > > -    int ret;
> > > +    uint64_t memory_sizes[8];
> > > +    void *cache_ptr;
> > > +    int i, ret, nregions;
> > >
> > >      if (!vub->chardev.chr) {
> > >          error_setg(errp, "vhost-user-base: missing chardev");
> > > @@ -314,7 +316,7 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
> > >
> > >      /* Allocate queues */
> > >      vub->vqs = g_ptr_array_sized_new(vub->num_vqs);
> > > -    for (int i = 0; i < vub->num_vqs; i++) {
> > > +    for (i = 0; i < vub->num_vqs; i++) {
> > >          g_ptr_array_add(vub->vqs,
> > >                          virtio_add_queue(vdev, vub->vq_size,
> > >                                           vub_handle_output));
> > > @@ -331,6 +333,37 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
> > >          do_vhost_user_cleanup(vdev, vub);
> >
> > Missing return statement.
>
> True, but this is unrelated to this patchset. I will fix it in a
> different patch, so that it can find its way in faster.
>
> >
> > >      }
> > >
> > > +    ret = vub->vhost_dev.vhost_ops->vhost_get_shmem_config(&vub->vhost_dev,
> > > +                                                           &nregions,
> > > +                                                           memory_sizes,
> >
> > Buffer overflow. vhost_get_shmem_config() copies out up to 256
> > memory_sizes[] elements. Please introduce a constant in the VIRTIO
> > header and use it instead of hardcoding uint64_t memory_sizes[8] above.
> >
> > > +                                                           errp);
> > > +
> > > +    if (ret < 0) {
> > > +        do_vhost_user_cleanup(vdev, vub);
> >
> > Missing return statement.
>
> Same here.

I'll correct myself here, this one was introduced in this patch, so is
not in mainline. Anyway, I think a goto may be a clearer pattern to
avoid missing the return statement.

> >
> > > +    }
> > > +
> > > +    for (i = 0; i < nregions; i++) {
> > > +        if (memory_sizes[i]) {
> > > +            if (memory_sizes[i] % qemu_real_host_page_size() != 0) {
> > > +                error_setg(errp, "Shared memory %d size must be a power of 2 "
> > > +                                 "no smaller than the page size", i);
> > > +                return;
> >
> > Missing do_vhost_user_cleanup().
>
> Maybe a goto would be preferable here? Just because the same exit
> pattern occurs quite a few times now.
>
> >
> > > +            }
> > > +
> > > +            cache_ptr = mmap(NULL, memory_sizes[i], PROT_NONE,
> > > +                            MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> >
> >
> >
> > > +            if (cache_ptr == MAP_FAILED) {
> > > +                error_setg_errno(errp, errno, "Unable to mmap blank cache");
> > > +                return;
> >
> > Missing do_vhost_user_cleanup().
> >
> > > +            }
> > > +
> > > +            virtio_new_shmem_region(vdev);
> > > +            memory_region_init_ram_ptr(vdev->shmem_list[i].mr,
> > > +                                       OBJECT(vdev), "vub-shm-" + i,
> > > +                                       memory_sizes[i], cache_ptr);
> >
> > I think memory_region_init_ram_ptr() is included in live migration, so
> > the contents of VIRTIO Shared Memory Regions will be transferred to the
> > destination QEMU and written to the equivalent memory region there. I'm
> > not sure this works:
> > 1. If there are PROT_NONE memory ranges, then live migration will
> >    probably crash the source QEMU while trying to send this memory to
> >    the destination QEMU.
> > 2. If the destination vhost-user device has not yet loaded its state and
> >    sent MAP messages setting up the VIRTIO Shared Memory Region, then
> >    receiving migrated data and writing it into this memory will fail.
> >
> > QEMU has a migration blocker API so that devices can refuse live
> > migration. For the time being a migration blocker is probably needed
> > here. See migrate_add_blocker()/migrate_del_blocker().
> >
> > > +        }
> > > +    }
> > > +
> > >      qemu_chr_fe_set_handlers(&vub->chardev, NULL, NULL, vub_event, NULL,
> > >                               dev, NULL, true);
> > >  }
> > > diff --git a/hw/virtio/vhost-user-device-pci.c b/hw/virtio/vhost-user-device-pci.c
> > > index efaf55d3dd..abf4e90c21 100644
> > > --- a/hw/virtio/vhost-user-device-pci.c
> > > +++ b/hw/virtio/vhost-user-device-pci.c
> > > @@ -8,14 +8,18 @@
> > >   */
> > >
> > >  #include "qemu/osdep.h"
> > > +#include "qapi/error.h"
> > >  #include "hw/qdev-properties.h"
> > >  #include "hw/virtio/vhost-user-base.h"
> > >  #include "hw/virtio/virtio-pci.h"
> > >
> > > +#define VIRTIO_DEVICE_PCI_CACHE_BAR 2
> >
> > "Cache" is ambigous. Call it shmem_bar here and everywhere else?
> >
> > > +
> > >  struct VHostUserDevicePCI {
> > >      VirtIOPCIProxy parent_obj;
> > >
> > >      VHostUserBase vub;
> > > +    MemoryRegion cachebar;
> > >  };
> > >
> > >  #define TYPE_VHOST_USER_DEVICE_PCI "vhost-user-device-pci-base"
> > > @@ -25,10 +29,39 @@ OBJECT_DECLARE_SIMPLE_TYPE(VHostUserDevicePCI, VHOST_USER_DEVICE_PCI)
> > >  static void vhost_user_device_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
> > >  {
> > >      VHostUserDevicePCI *dev = VHOST_USER_DEVICE_PCI(vpci_dev);
> > > -    DeviceState *vdev = DEVICE(&dev->vub);
> > > -
> > > +    DeviceState *dev_state = DEVICE(&dev->vub);
> > > +    VirtIODevice *vdev = VIRTIO_DEVICE(dev_state);
> > > +    MemoryRegion *mr;
> > > +    uint64_t offset = 0, cache_size = 0;
> > > +    int i;
> > > +
> > >      vpci_dev->nvectors = 1;
> > > -    qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
> > > +    qdev_realize(dev_state, BUS(&vpci_dev->bus), errp);
> > > +
> > > +    for (i = 0; i < vdev->n_shmem_regions; i++) {
> > > +        mr = vdev->shmem_list[i].mr;
> > > +        if (mr->size > UINT64_MAX - cache_size) {
> > > +            error_setg(errp, "Total shared memory required overflow");
> > > +            return;
> > > +        }
> > > +        cache_size = cache_size + mr->size;
> > > +    }
> > > +    if (cache_size) {
> > > +        memory_region_init(&dev->cachebar, OBJECT(vpci_dev),
> > > +                           "vhost-device-pci-cachebar", cache_size);
> > > +        for (i = 0; i < vdev->n_shmem_regions; i++) {
> > > +            mr = vdev->shmem_list[i].mr;
> > > +            memory_region_add_subregion(&dev->cachebar, offset, mr);
> > > +            virtio_pci_add_shm_cap(vpci_dev, VIRTIO_DEVICE_PCI_CACHE_BAR,
> > > +                                   offset, mr->size, i);
> > > +            offset = offset + mr->size;
> > > +        }
> > > +        pci_register_bar(&vpci_dev->pci_dev, VIRTIO_DEVICE_PCI_CACHE_BAR,
> > > +                        PCI_BASE_ADDRESS_SPACE_MEMORY |
> > > +                        PCI_BASE_ADDRESS_MEM_PREFETCH |
> > > +                        PCI_BASE_ADDRESS_MEM_TYPE_64,
> > > +                        &dev->cachebar);
> > > +    }
> > >  }
> > >
> > >  static void vhost_user_device_pci_class_init(ObjectClass *klass, void *data)
> > > --
> > > 2.45.2
> > >