Re: [Question mpam mpam/snapshot+extras/v6.18-rc1] Question with Configuring iommu_group in 'task'

[Qinxin Xia]

On 2026/3/27 18:47:49, Ben Horgan <ben.horgan@xxxxxxx> wrote:
> Hi Qinxin,
>
> On 3/27/26 10:21, Qinxin Xia wrote:
> > Hello everyone!
> >
> > In earlier versions, mpam supports the configuration of iommu_groups.
> >
> >   823 static ssize_t rdtgroup_tasks_write(struct kernfs_open_file *of,
> >   824                                     char *buf, size_t nbytes,
> > loff_t off)
> >   825 {
> >   826         struct rdtgroup *rdtgrp;
> >   827         int iommu_group_id;
> >   828         bool is_iommu;
> >   829         char *pid_str;
> >   830         int ret = 0;
> >   831         pid_t pid;
> >   832
> >   833         rdtgrp = rdtgroup_kn_lock_live(of->kn);
> >   834         if (!rdtgrp) {
> >   835                 rdtgroup_kn_unlock(of->kn);
> >   836                 return -ENOENT;
> >   837         }
> >   838         rdt_last_cmd_clear();
> >   839
> >   840         if (rdtgrp->mode == RDT_MODE_PSEUDO_LOCKED ||
> >   841             rdtgrp->mode == RDT_MODE_PSEUDO_LOCKSETUP) {
> >   842                 ret = -EINVAL;
> >   843                 rdt_last_cmd_puts("Pseudo-locking in progress\n");
> >   844                 goto unlock;
> >   845         }
> >   846
> >   847         while (buf && buf[0] != '\0' && buf[0] != '\n') {
> >   848                 pid_str = strim(strsep(&buf, ","));
> >   849
> >   850                 is_iommu = string_is_iommu_group(pid_str, &iommu_group_id);
> >
> > What puzzles me is why we would put it under 'task'—this seems a little
> >   strange to users.It seems they are not related.Why don't we add a new
> > interface like 'iommu'?
>
> I think it is likely that this interface would change if upstream support is added.

I have done some work in this direction before, and I will release an
RFC later for further discussion.:-)

> >   851                 if (is_iommu)
> >   852                         ret = rdtgroup_move_iommu(iommu_group_id, rdtgrp, of);
> >   853                 else if (kstrtoint(pid_str, 0, &pid)) {
> >   854                         rdt_last_cmd_printf("Task list parsing error pid %s\n", pid_str);
> >   855                         ret = -EINVAL;
> >   856                         break;
> >   857                 }
> >   858
> >   859                 if (pid < 0) {
> >   860                         rdt_last_cmd_printf("Invalid pid %d\n", pid);
> >   861                         ret = -EINVAL;
> >   862                         break;
> >   863                 }
> >   864
> >
> > In future glue versions, will you re-enable support for iommu_group, and
> > if so, will the configuration scheme be changed?
>
> Please can you let us know about your usecase so that we can get more information to decide
> what the best interface would be?
>
> Thanks,
>
> Ben

We want to use the iommu mpam to implement stream control for different
PCIe devices. By limiting the access bandwidth of some PCIe devices, the
high-priority service devices can obtain more resources.

--
Thanks,
Qinxin