Date:   Mon, 12 Dec 2022 11:40:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>
Cc:     stefanha@...hat.com, mst@...hat.com, sgarzare@...hat.com,
        eperezma@...hat.com, cohuck@...hat.com, arei.gonglei@...wei.com,
        yechuan@...wei.com, huangzhichao@...wei.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vdpasim: support doorbell mapping

On Mon, Dec 12, 2022 at 8:51 AM Longpeng (Mike, Cloud Infrastructure
Service Product Dept.) <longpeng2@...wei.com> wrote:
>
>
>
> 在 2022/12/8 15:37, Jason Wang 写道:
> > On Tue, Dec 6, 2022 at 4:46 PM Longpeng (Mike, Cloud Infrastructure
> > Service Product Dept.) <longpeng2@...wei.com> wrote:
> >>
> >>
> >>
> >> 在 2022/12/6 14:59, Jason Wang 写道:
> >>> On Mon, Dec 5, 2022 at 9:52 PM Longpeng(Mike) <longpeng2@...wei.com> wrote:
> >>>>
> >>>> From: Longpeng <longpeng2@...wei.com>
> >>>>
> >>>> Support doorbell mapping for vdpasim devices, then we can test the notify
> >>>> passthrough feature even if there's no real hardware on hand.
> >>>>
> >>>> Allocates a dummy page which is used to emulate the notify page of the device,
> >>>> all VQs share the same notify register  that initiated to 0xffff. A  periodic
> >>>> work will check whether there're requests need to process ( the value of the
> >>>> notify register is 0xffff or not ).
> >>>>
> >>>> This cap is disabled as default, users can enable it as follow:
> >>>>     modprobe vdpa_sim notify_passthrough=true
> >>>>
> >>>> Signed-off-by: Longpeng <longpeng2@...wei.com>
> >>>> ---
> >>>> Changes v1->v2:
> >>>>     - support both kick mode and passthrough mode. [Jason]
> >>>>     - poll the notify register first. [Jason, Michael]
> >>>> ---
> >>>>    drivers/vdpa/vdpa_sim/vdpa_sim.c | 77 ++++++++++++++++++++++++++++++++
> >>>>    drivers/vdpa/vdpa_sim/vdpa_sim.h |  3 ++
> >>>>    2 files changed, 80 insertions(+)
> >>>>
> >>>> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> >>>> index 7438a89ce939..07096a04dabb 100644
> >>>> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
> >>>> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> >>>> @@ -14,6 +14,7 @@
> >>>>    #include <linux/slab.h>
> >>>>    #include <linux/sched.h>
> >>>>    #include <linux/dma-map-ops.h>
> >>>> +#include <asm/set_memory.h>
> >>>>    #include <linux/vringh.h>
> >>>>    #include <linux/vdpa.h>
> >>>>    #include <linux/vhost_iotlb.h>
> >>>> @@ -36,9 +37,16 @@ module_param(max_iotlb_entries, int, 0444);
> >>>>    MODULE_PARM_DESC(max_iotlb_entries,
> >>>>                    "Maximum number of iotlb entries for each address space. 0 means unlimited. (default: 2048)");
> >>>>
> >>>> +static bool notify_passthrough;
> >>>> +module_param(notify_passthrough, bool, 0444);
> >>>> +MODULE_PARM_DESC(notify_passthrough,
> >>>> +                "Enable vq notify(doorbell) area mapping. (default: false)");
> >>>
> >>> I'm not sure if it's worth doing this, I think we can afford the cost
> >>> of periodic work (especially considering it's a simulator).
> >>>
> >> We can support kick mode and passthgourh mode simultaneously now, it is
> >> a little unnecessary. I would remove it in next version.
> >>
> >>>> +
> >>>>    #define VDPASIM_QUEUE_ALIGN PAGE_SIZE
> >>>>    #define VDPASIM_QUEUE_MAX 256
> >>>>    #define VDPASIM_VENDOR_ID 0
> >>>> +#define VDPASIM_VRING_POLL_PERIOD 100 /* ms */
> >>>> +#define VDPASIM_NOTIFY_DEFVAL 0xffff
> >>>>
> >>>>    static struct vdpasim *vdpa_to_sim(struct vdpa_device *vdpa)
> >>>>    {
> >>>> @@ -246,6 +254,28 @@ static const struct dma_map_ops vdpasim_dma_ops = {
> >>>>    static const struct vdpa_config_ops vdpasim_config_ops;
> >>>>    static const struct vdpa_config_ops vdpasim_batch_config_ops;
> >>>>
> >>>> +static void vdpasim_notify_work(struct work_struct *work)
> >>>> +{
> >>>> +       struct vdpasim *vdpasim;
> >>>> +       u16 *val;
> >>>> +
> >>>> +       vdpasim = container_of(work, struct vdpasim, notify_work.work);
> >>>> +
> >>>> +       if (!(vdpasim->status & VIRTIO_CONFIG_S_DRIVER_OK))
> >>>> +               goto out;
> >>>> +
> >>>> +       if (!vdpasim->running)
> >>>> +               goto out;
> >>>> +
> >>>> +       val = (u16 *)vdpasim->notify;
> >>>> +       if (xchg(val, VDPASIM_NOTIFY_DEFVAL) != VDPASIM_NOTIFY_DEFVAL)
> >>>> +               schedule_work(&vdpasim->work);
> >>>> +
> >>>> +out:
> >>>> +       schedule_delayed_work(&vdpasim->notify_work,
> >>>> +                             msecs_to_jiffies(VDPASIM_VRING_POLL_PERIOD));
> >>>> +}
> >>>> +
> >>>>    struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *dev_attr,
> >>>>                                  const struct vdpa_dev_set_config *config)
> >>>>    {
> >>>> @@ -287,6 +317,18 @@ struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *dev_attr,
> >>>>           set_dma_ops(dev, &vdpasim_dma_ops);
> >>>>           vdpasim->vdpa.mdev = dev_attr->mgmt_dev;
> >>>>
> >>>> +       if (notify_passthrough) {
> >>>> +               INIT_DELAYED_WORK(&vdpasim->notify_work, vdpasim_notify_work);
> >>>> +
> >>>> +               vdpasim->notify = __get_free_page(GFP_KERNEL | __GFP_ZERO);
> >>>> +               if (!vdpasim->notify)
> >>>> +                       goto err_iommu;
> >>>> +#ifdef CONFIG_X86
> >>>> +               set_memory_uc(vdpasim->notify, 1);
> >>>> +#endif
> >>>
> >>> I had the same question with version 1, any reason for having this
> >>> part uncacheable? It's a hint that we have bugs somewhere. Are we
> >>> missing ACCESS/ORDER_PLATFORM or other features?
> >>>
> >> Oh, I had replied you in that thread.
> >>
> >> The vm_page_prot of notify mapping VMA is pgprot_noncached (see
> >> vhost_vdpa_fault) but the vdpasim->notify is WB, so we should set its
> >> memtype to UC here and set it back to WB when releasing the device.
> >
> > Ok, but any reason to have a X86 guard for this? This basically makes
> > it won't work for non-x86 archs?
> >
> Because only X86 defines __HAVE_PFNMAP_TRACKING, track_pfn_remap does
> nothing on other archs.

Ok, but this seems fragile, as we don't have a guarantee that it won't
be implemented in the future.

>
> > I think the correct way is to introduce a new config_ops to fetch the
> > correct pgprot. Then we are fine.
> >
> Is it worth doing this?

I think it's worth since it:

1) only require small tweak on the vdpa core
2) avoid arch specific code in the simulator

> It seems the vdpasim device is just a special
> case (using RAM to emulates the notify area) and other real HW devices
> work fine with the current implementation?

There's no guarantee that we won't have any other software based vDPA
devices. We've already had a simulator and VDUSE. And we should not
exclude the possibility of another one.

Thanks

>
> > Thanks
> >
> >>
> >> Otherwise, the warning in trace_pfn_remap() would be triggered.
> >> For example:
> >> x86/PAT: CPU 16/KVM:17819 map pfn RAM range req uncached-minus for [mem
> >> 0x5151f3000-0x5151f3fff], got write-back
> >>
> >>>> +               *(u16 *)vdpasim->notify = VDPASIM_NOTIFY_DEFVAL;
> >>>
> >>> WRITE_ONCE()?
> >>>
> >> Okay.
> >>
> >>>> +       }
> >>>> +
> >>>>           vdpasim->config = kzalloc(dev_attr->config_size, GFP_KERNEL);
> >>>>           if (!vdpasim->config)
> >>>>                   goto err_iommu;
> >>>> @@ -495,6 +537,18 @@ static u8 vdpasim_get_status(struct vdpa_device *vdpa)
> >>>>           return status;
> >>>>    }
> >>>>
> >>>> +static void vdpasim_set_notify_work(struct vdpasim *vdpasim, bool start)
> >>>> +{
> >>>> +       if (!notify_passthrough)
> >>>> +               return;
> >>>
> >>> Only two callers for this function: one is start another is stop. If
> >>> we decide to get rid of notify_passthrough, I'd rather opencode the
> >>> schedule/cancel_delayed().
> >>>
> >> Okay, thanks.
> >>
> >>> Thanks
> >>>
> >>>> +
> >>>> +       if (start)
> >>>> +               schedule_delayed_work(&vdpasim->notify_work,
> >>>> +                                     msecs_to_jiffies(VDPASIM_VRING_POLL_PERIOD));
> >>>> +       else
> >>>> +               cancel_delayed_work_sync(&vdpasim->notify_work);
> >>>> +}
> >>>> +
> >>>>    static void vdpasim_set_status(struct vdpa_device *vdpa, u8 status)
> >>>>    {
> >>>>           struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
> >>>> @@ -502,12 +556,14 @@ static void vdpasim_set_status(struct vdpa_device *vdpa, u8 status)
> >>>>           spin_lock(&vdpasim->lock);
> >>>>           vdpasim->status = status;
> >>>>           spin_unlock(&vdpasim->lock);
> >>>> +       vdpasim_set_notify_work(vdpasim, status & VIRTIO_CONFIG_S_DRIVER_OK);
> >>>>    }
> >>>>
> >>>>    static int vdpasim_reset(struct vdpa_device *vdpa, bool clear)
> >>>>    {
> >>>>           struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
> >>>>
> >>>> +       vdpasim_set_notify_work(vdpasim, false);
> >>>>           spin_lock(&vdpasim->lock);
> >>>>           vdpasim->status = 0;
> >>>>           vdpasim_do_reset(vdpasim);
> >>>> @@ -672,11 +728,24 @@ static int vdpasim_dma_unmap(struct vdpa_device *vdpa, unsigned int asid,
> >>>>           return 0;
> >>>>    }
> >>>>
> >>>> +static struct vdpa_notification_area
> >>>> +vdpasim_get_vq_notification(struct vdpa_device *vdpa, u16 qid)
> >>>> +{
> >>>> +       struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
> >>>> +       struct vdpa_notification_area notify;
> >>>> +
> >>>> +       notify.addr = virt_to_phys((void *)vdpasim->notify);
> >>>> +       notify.size = PAGE_SIZE;
> >>>> +
> >>>> +       return notify;
> >>>> +}
> >>>> +
> >>>>    static void vdpasim_free(struct vdpa_device *vdpa)
> >>>>    {
> >>>>           struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
> >>>>           int i;
> >>>>
> >>>> +       vdpasim_set_notify_work(vdpasim, false);
> >>>>           cancel_work_sync(&vdpasim->work);
> >>>>
> >>>>           for (i = 0; i < vdpasim->dev_attr.nvqs; i++) {
> >>>> @@ -693,6 +762,12 @@ static void vdpasim_free(struct vdpa_device *vdpa)
> >>>>           vhost_iotlb_free(vdpasim->iommu);
> >>>>           kfree(vdpasim->vqs);
> >>>>           kfree(vdpasim->config);
> >>>> +       if (vdpasim->notify) {
> >>>> +#ifdef CONFIG_X86
> >>>> +               set_memory_wb(vdpasim->notify, 1);
> >>>> +#endif
> >>>> +               free_page(vdpasim->notify);
> >>>> +       }
> >>>>    }
> >>>>
> >>>>    static const struct vdpa_config_ops vdpasim_config_ops = {
> >>>> @@ -704,6 +779,7 @@ static const struct vdpa_config_ops vdpasim_config_ops = {
> >>>>           .get_vq_ready           = vdpasim_get_vq_ready,
> >>>>           .set_vq_state           = vdpasim_set_vq_state,
> >>>>           .get_vq_state           = vdpasim_get_vq_state,
> >>>> +       .get_vq_notification    = vdpasim_get_vq_notification,
> >>>>           .get_vq_align           = vdpasim_get_vq_align,
> >>>>           .get_vq_group           = vdpasim_get_vq_group,
> >>>>           .get_device_features    = vdpasim_get_device_features,
> >>>> @@ -737,6 +813,7 @@ static const struct vdpa_config_ops vdpasim_batch_config_ops = {
> >>>>           .get_vq_ready           = vdpasim_get_vq_ready,
> >>>>           .set_vq_state           = vdpasim_set_vq_state,
> >>>>           .get_vq_state           = vdpasim_get_vq_state,
> >>>> +       .get_vq_notification    = vdpasim_get_vq_notification,
> >>>>           .get_vq_align           = vdpasim_get_vq_align,
> >>>>           .get_vq_group           = vdpasim_get_vq_group,
> >>>>           .get_device_features    = vdpasim_get_device_features,
> >>>> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.h b/drivers/vdpa/vdpa_sim/vdpa_sim.h
> >>>> index 0e78737dcc16..0769ccbd3911 100644
> >>>> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.h
> >>>> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.h
> >>>> @@ -69,6 +69,9 @@ struct vdpasim {
> >>>>           bool running;
> >>>>           /* spinlock to synchronize iommu table */
> >>>>           spinlock_t iommu_lock;
> >>>> +       /* dummy notify page */
> >>>> +       unsigned long notify;
> >>>> +       struct delayed_work notify_work;
> >>>>    };
> >>>>
> >>>>    struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *attr,
> >>>> --
> >>>> 2.23.0
> >>>>
> >>>
> >>> .
> >>
> >
> > .
>

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