Re: [PATCH v5 14/29] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC ioctl

Nicolin Chen
2025-05-30T18:42:23Z

On Fri, May 30, 2025 at 03:25:19PM -0300, Jason Gunthorpe wrote:
> On Fri, May 30, 2025 at 11:23:02AM -0700, Nicolin Chen wrote:
> > On Fri, May 30, 2025 at 02:40:37PM -0300, Jason Gunthorpe wrote:
> > > On Fri, May 30, 2025 at 10:38:24AM -0700, Nicolin Chen wrote:
> > > > On Fri, May 30, 2025 at 01:14:55PM -0300, Jason Gunthorpe wrote:
> > > > > On Sat, May 17, 2025 at 08:21:31PM -0700, Nicolin Chen wrote:
> > > > > > + offset =
> > > > > > + cmd->nesting_parent_iova - PAGE_ALIGN(cmd->nesting_parent_iova);
> > > > > > + max_npages = DIV_ROUND_UP(offset + cmd->length, PAGE_SIZE);
> > > > >
> > > > > This should probably be capped to PAGE_SIZE/sizeof(void *), return
> > > > > EINVAL if not
> > > >
> > > > Hmm, mind elaborating where this PAGE_SIZE/sizeof comes from?
> > >
> > > We can usually allocate up to a PAGE_SIZE without too much
> > > trouble. Beyond that it gets more likely to fail.
> >
> > If PAGE_SIZE=4096, the upper limit for max_npages is 512, i.e. the
> > max size of a guest queue is 2MB? It seems to be too small, as the
> > VMM can use a larger huge page size to back the guest queue?
>
> May need to make a new API that returns a bio_vec or something else
> more efficient then :\

Hmm, that sounds like a rabbit hole :-\

Let me leave a FIXME at this max_npages calculation instead..

Thanks
Nicolin