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Message-ID: <CACGkMEtavEOcWKaHZGdHD5MH94dvL=f8qe-kUS==bPB66i=-yw@mail.gmail.com>
Date: Mon, 17 Nov 2025 12:23:11 +0800
From: Jason Wang <jasowang@...hat.com>
To: Eugenio Perez Martin <eperezma@...hat.com>
Cc: "Michael S . Tsirkin" <mst@...hat.com>, Laurent Vivier <lvivier@...hat.com>, virtualization@...ts.linux.dev,
Maxime Coquelin <mcoqueli@...hat.com>, Cindy Lu <lulu@...hat.com>, linux-kernel@...r.kernel.org,
Yongji Xie <xieyongji@...edance.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
Stefano Garzarella <sgarzare@...hat.com>
Subject: Re: [PATCH v9 5/6] vduse: add vq group asid support
On Fri, Nov 14, 2025 at 7:25 PM Eugenio Perez Martin
<eperezma@...hat.com> wrote:
>
> On Fri, Nov 14, 2025 at 1:55 AM Jason Wang <jasowang@...hat.com> wrote:
> >
> > On Thu, Nov 13, 2025 at 7:56 PM Eugenio Pérez <eperezma@...hat.com> wrote:
> > >
> > > Add support for assigning Address Space Identifiers (ASIDs) to each VQ
> > > group. This enables mapping each group into a distinct memory space.
> > >
> > > Now that the driver can change ASID in the middle of operation, the
> > > domain that each vq address point is also protected by domain_lock.
> >
> > Maybe it's better to document what is protected by RCU and how.
> >
>
> I added the _rcu annotation but I can expand it for sure. I can also
> modify the commit message.
>
> > More below.
> >
> > >
> > > Acked-by: Jason Wang <jasowang@...hat.com>
>
> I forgot to remove this, my bad!
>
> > > Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> > > ---
> > > v9:
> > > * Replace mutex with RCU, as the vdpa map_ops can run from atomic
> > > context.
> > >
> > > v8:
> > > * Revert the mutex to rwlock change, it needs proper profiling to
> > > justify it.
> > >
> > > v7:
> > > * Take write lock in the error path (Jason).
> > >
> > > v6:
> > > * Make vdpa_dev_add use gotos for error handling (MST).
> > > * s/(dev->api_version < 1) ?/(dev->api_version < VDUSE_API_VERSION_1) ?/
> > > (MST).
> > > * Fix struct name not matching in the doc.
> > >
> > > v5:
> > > * Properly return errno if copy_to_user returns >0 in VDUSE_IOTLB_GET_FD
> > > ioctl (Jason).
> > > * Properly set domain bounce size to divide equally between nas (Jason).
> > > * Exclude "padding" member from the only >V1 members in
> > > vduse_dev_request.
> > >
> > > v4:
> > > * Divide each domain bounce size between the device bounce size (Jason).
> > > * revert unneeded addr = NULL assignment (Jason)
> > > * Change if (x && (y || z)) return to if (x) { if (y) return; if (z)
> > > return; } (Jason)
> > > * Change a bad multiline comment, using @ caracter instead of * (Jason).
> > > * Consider config->nas == 0 as a fail (Jason).
> > >
> > > v3:
> > > * Get the vduse domain through the vduse_as in the map functions
> > > (Jason).
> > > * Squash with the patch creating the vduse_as struct (Jason).
> > > * Create VDUSE_DEV_MAX_AS instead of comparing agains a magic number
> > > (Jason)
> > >
> > > v2:
> > > * Convert the use of mutex to rwlock.
> > >
> > > RFC v3:
> > > * Increase VDUSE_MAX_VQ_GROUPS to 0xffff (Jason). It was set to a lower
> > > value to reduce memory consumption, but vqs are already limited to
> > > that value and userspace VDUSE is able to allocate that many vqs.
> > > * Remove TODO about merging VDUSE_IOTLB_GET_FD ioctl with
> > > VDUSE_IOTLB_GET_INFO.
> > > * Use of array_index_nospec in VDUSE device ioctls.
> > > * Embed vduse_iotlb_entry into vduse_iotlb_entry_v2.
> > > * Move the umem mutex to asid struct so there is no contention between
> > > ASIDs.
> > >
> > > RFC v2:
> > > * Make iotlb entry the last one of vduse_iotlb_entry_v2 so the first
> > > part of the struct is the same.
> > > ---
> > > drivers/vdpa/vdpa_user/vduse_dev.c | 370 ++++++++++++++++++++---------
> > > include/uapi/linux/vduse.h | 53 ++++-
> > > 2 files changed, 314 insertions(+), 109 deletions(-)
> > >
> > > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > index 97be04f73fbf..ff95ed56f22d 100644
> > > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> > > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > @@ -11,6 +11,7 @@
> > > #include "linux/virtio_net.h"
> > > #include <linux/init.h>
> > > #include <linux/module.h>
> > > +#include <linux/rcupdate.h>
> > > #include <linux/cdev.h>
> > > #include <linux/device.h>
> > > #include <linux/eventfd.h>
> > > @@ -41,6 +42,7 @@
> > >
> > > #define VDUSE_DEV_MAX (1U << MINORBITS)
> > > #define VDUSE_DEV_MAX_GROUPS 0xffff
> > > +#define VDUSE_DEV_MAX_AS 0xffff
> > > #define VDUSE_MAX_BOUNCE_SIZE (1024 * 1024 * 1024)
> > > #define VDUSE_MIN_BOUNCE_SIZE (1024 * 1024)
> > > #define VDUSE_BOUNCE_SIZE (64 * 1024 * 1024)
> > > @@ -86,7 +88,14 @@ struct vduse_umem {
> > > struct mm_struct *mm;
> > > };
> > >
> > > +struct vduse_as {
> > > + struct vduse_iova_domain *domain;
> > > + struct vduse_umem *umem;
> > > + struct mutex mem_lock;
> > > +};
> > > +
> > > struct vduse_vq_group {
> > > + struct vduse_as *as __rcu;
> > > struct vduse_dev *dev;
> > > };
> > >
> > > @@ -94,7 +103,7 @@ struct vduse_dev {
> > > struct vduse_vdpa *vdev;
> > > struct device *dev;
> > > struct vduse_virtqueue **vqs;
> > > - struct vduse_iova_domain *domain;
> > > + struct vduse_as *as;
> > > char *name;
> > > struct mutex lock;
> > > spinlock_t msg_lock;
> > > @@ -122,9 +131,8 @@ struct vduse_dev {
> > > u32 vq_num;
> > > u32 vq_align;
> > > u32 ngroups;
> > > - struct vduse_umem *umem;
> > > + u32 nas;
> > > struct vduse_vq_group *groups;
> > > - struct mutex mem_lock;
> > > unsigned int bounce_size;
> > > struct mutex domain_lock;
> > > };
> > > @@ -314,7 +322,7 @@ static int vduse_dev_set_status(struct vduse_dev *dev, u8 status)
> > > return vduse_dev_msg_sync(dev, &msg);
> > > }
> > >
> > > -static int vduse_dev_update_iotlb(struct vduse_dev *dev,
> > > +static int vduse_dev_update_iotlb(struct vduse_dev *dev, u32 asid,
> > > u64 start, u64 last)
> > > {
> > > struct vduse_dev_msg msg = { 0 };
> > > @@ -323,8 +331,14 @@ static int vduse_dev_update_iotlb(struct vduse_dev *dev,
> > > return -EINVAL;
> > >
> > > msg.req.type = VDUSE_UPDATE_IOTLB;
> > > - msg.req.iova.start = start;
> > > - msg.req.iova.last = last;
> > > + if (dev->api_version < VDUSE_API_VERSION_1) {
> > > + msg.req.iova.start = start;
> > > + msg.req.iova.last = last;
> > > + } else {
> > > + msg.req.iova_v2.start = start;
> > > + msg.req.iova_v2.last = last;
> > > + msg.req.iova_v2.asid = asid;
> > > + }
> > >
> > > return vduse_dev_msg_sync(dev, &msg);
> > > }
> > > @@ -436,14 +450,32 @@ static __poll_t vduse_dev_poll(struct file *file, poll_table *wait)
> > > return mask;
> > > }
> > >
> > > +/* Force set the asid to a vq group without a message to the VDUSE device */
> > > +static void vduse_set_group_asid_nomsg(struct vduse_dev *dev,
> > > + unsigned int group, unsigned int asid)
> > > +{
> > > + /*
> > > + * Two concurrent updates to this pointer are valid as they cannot
> > > + * point to an invalid region. It is ok for them to race as long as
> > > + * the readers see a consistent state through RCU.
> > > + */
> > > + rcu_assign_pointer(dev->groups[group].as, &dev->as[asid]);
> >
> > I'd expect at least a synchronize_rcu() here to wait for the read is done?
> >
>
> What's the use? The only thing left here is to return from
> vduse_set_group_asid_nomsg, and we don't need to wait for readers
> here, do we?
See below.
>
> > > +}
> > > +
> > > static void vduse_dev_reset(struct vduse_dev *dev)
> > > {
> > > int i;
> > > - struct vduse_iova_domain *domain = dev->domain;
> > >
> > > /* The coherent mappings are handled in vduse_dev_free_coherent() */
> > > - if (domain && domain->bounce_map)
> > > - vduse_domain_reset_bounce_map(domain);
> > > + for (i = 0; i < dev->nas; i++) {
> > > + struct vduse_iova_domain *domain = dev->as[i].domain;
> > > +
> > > + if (domain && domain->bounce_map)
> > > + vduse_domain_reset_bounce_map(domain);
> > > + }
> > > +
> > > + for (i = 0; i < dev->ngroups; i++)
> > > + vduse_set_group_asid_nomsg(dev, i, 0);
> > >
> > > down_write(&dev->rwsem);
> > >
> > > @@ -623,6 +655,29 @@ static union virtio_map vduse_get_vq_map(struct vdpa_device *vdpa, u16 idx)
> > > return ret;
> > > }
> > >
> > > +static int vduse_set_group_asid(struct vdpa_device *vdpa, unsigned int group,
> > > + unsigned int asid)
> > > +{
> > > + struct vduse_dev *dev = vdpa_to_vduse(vdpa);
> > > + struct vduse_dev_msg msg = { 0 };
> > > + int r;
> > > +
> > > + if (dev->api_version < VDUSE_API_VERSION_1 ||
> > > + group >= dev->ngroups || asid >= dev->nas)
> > > + return -EINVAL;
> > > +
> > > + msg.req.type = VDUSE_SET_VQ_GROUP_ASID;
> > > + msg.req.vq_group_asid.group = group;
> > > + msg.req.vq_group_asid.asid = asid;
> > > +
> > > + r = vduse_dev_msg_sync(dev, &msg);
> > > + if (r < 0)
> > > + return r;
> > > +
> > > + vduse_set_group_asid_nomsg(dev, group, asid);
> > > + return 0;
> > > +}
> > > +
> > > static int vduse_vdpa_get_vq_state(struct vdpa_device *vdpa, u16 idx,
> > > struct vdpa_vq_state *state)
> > > {
> > > @@ -794,13 +849,13 @@ static int vduse_vdpa_set_map(struct vdpa_device *vdpa,
> > > struct vduse_dev *dev = vdpa_to_vduse(vdpa);
> > > int ret;
> > >
> > > - ret = vduse_domain_set_map(dev->domain, iotlb);
> > > + ret = vduse_domain_set_map(dev->as[asid].domain, iotlb);
> > > if (ret)
> > > return ret;
> > >
> > > - ret = vduse_dev_update_iotlb(dev, 0ULL, ULLONG_MAX);
> > > + ret = vduse_dev_update_iotlb(dev, asid, 0ULL, ULLONG_MAX);
> > > if (ret) {
> > > - vduse_domain_clear_map(dev->domain, iotlb);
> > > + vduse_domain_clear_map(dev->as[asid].domain, iotlb);
> > > return ret;
> > > }
> > >
> > > @@ -843,6 +898,7 @@ static const struct vdpa_config_ops vduse_vdpa_config_ops = {
> > > .get_vq_affinity = vduse_vdpa_get_vq_affinity,
> > > .reset = vduse_vdpa_reset,
> > > .set_map = vduse_vdpa_set_map,
> > > + .set_group_asid = vduse_set_group_asid,
> > > .get_vq_map = vduse_get_vq_map,
> > > .free = vduse_vdpa_free,
> > > };
> > > @@ -852,14 +908,17 @@ static void vduse_dev_sync_single_for_device(union virtio_map token,
> > > enum dma_data_direction dir)
> > > {
> > > struct vduse_dev *vdev;
> > > + struct vduse_as *as;
> > > struct vduse_iova_domain *domain;
> > >
> > > if (!token.group)
> > > return;
> > >
> > > vdev = token.group->dev;
> > > - domain = vdev->domain;
> > > -
> > > + rcu_read_lock();
> > > + as = rcu_dereference(token.group->as);
> > > + domain = as->domain;
> > > + rcu_read_unlock();
> > > vduse_domain_sync_single_for_device(domain, dma_addr, size, dir);
> >
> > This is suspicious, at least we should do rcu_read_unlock() after
> > vduse_domain_sync_single_for_device(), otherwise I don't see how RCU
> > works.
> >
>
> RCU is protecting that the address space pointer of the vq group is
> not modified concurrently with the access. Ideally, this should be a
> full lock, but just making sure that all accesses from the reader are
> coherent is enough. Userspace should expect nothing if it uses the map
> and modifies the vq group ASID at the same time anyway, but the kernel
> needs to be sure that it does not see intermediate states. TBH, we
> could move to a READ_ONCE / WRITE_ONCE, would that be more clear?
Using READ_ONCE/WRITE_ONCE() needs to make sure the ordering is
handled correctly.
But I meant what happens if
[cpu0]rcu_read_lock()
[cpu0]as = rcu_dereference(token.group->as)
[cpu0]...
[cpu0]rcu_read_unlock()
[cpu1]rcu_assign_pointer(token.group->as)
[cpu0]vduse_domain_sync_single_for_device()
If this is not an issue, RCU is not a must, but please explain why.
If this is an issue, we need to fix it.
It's basically a question that
1) should we need to wait for the DMA to be completed before assigning
to the new as
2) should we track the set_group_asid() for the group that has pending
DMA to avoid potential issue
>
> The function vduse_domain uses its own lock to protect concurrent
> access to the maps of the ASID itself, as they were needed before
> implementing ASID already.
Thanks
>
> > > }
> > >
> > > @@ -868,14 +927,17 @@ static void vduse_dev_sync_single_for_cpu(union virtio_map token,
> > > enum dma_data_direction dir)
> > > {
> > > struct vduse_dev *vdev;
> > > + struct vduse_as *as;
> > > struct vduse_iova_domain *domain;
> > >
> > > if (!token.group)
> > > return;
> > >
> > > vdev = token.group->dev;
> > > - domain = vdev->domain;
> > > -
> > > + rcu_read_lock();
> > > + as = rcu_dereference(token.group->as);
> > > + domain = as->domain;
> > > + rcu_read_unlock();
> > > vduse_domain_sync_single_for_cpu(domain, dma_addr, size, dir);
> > > }
> > >
> > > @@ -885,15 +947,21 @@ static dma_addr_t vduse_dev_map_page(union virtio_map token, struct page *page,
> > > unsigned long attrs)
> > > {
> > > struct vduse_dev *vdev;
> > > + struct vduse_as *as;
> > > struct vduse_iova_domain *domain;
> > > + dma_addr_t r;
> > >
> > > if (!token.group)
> > > return DMA_MAPPING_ERROR;
> > >
> > > vdev = token.group->dev;
> > > - domain = vdev->domain;
> > > + rcu_read_lock();
> > > + as = rcu_dereference(token.group->as);
> > > + domain = as->domain;
> > > + rcu_read_unlock();
> > > + r = vduse_domain_map_page(domain, page, offset, size, dir, attrs);
> >
> > Same here.
> >
> > Thanks
> >
>
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