Re: [PATCH RFCv2 09/13] iommufd: Add IOMMU_OPTION_SW_MSI_START/SIZE ioctls

[Nicolin Chen]

On Thu, Jan 23, 2025 at 10:07:13AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > Sent: Saturday, January 11, 2025 11:32 AM
> > 
> > @@ -294,7 +294,9 @@ struct iommu_ioas_unmap {
> > 
> >  /**
> >   * enum iommufd_option - ioctl(IOMMU_OPTION_RLIMIT_MODE) and
> > - *                       ioctl(IOMMU_OPTION_HUGE_PAGES)
> > + *                       ioctl(IOMMU_OPTION_HUGE_PAGES) and
> > + *                       ioctl(IOMMU_OPTION_SW_MSI_START) and
> > + *                       ioctl(IOMMU_OPTION_SW_MSI_SIZE)
> >   * @IOMMU_OPTION_RLIMIT_MODE:
> >   *    Change how RLIMIT_MEMLOCK accounting works. The caller must have
> > privilege
> >   *    to invoke this. Value 0 (default) is user based accounting, 1 uses process
> > @@ -304,10 +306,24 @@ struct iommu_ioas_unmap {
> >   *    iommu mappings. Value 0 disables combining, everything is mapped to
> >   *    PAGE_SIZE. This can be useful for benchmarking.  This is a per-IOAS
> >   *    option, the object_id must be the IOAS ID.
> > + * @IOMMU_OPTION_SW_MSI_START:
> > + *    Change the base address of the IOMMU mapping region for MSI
> > doorbell(s).
> > + *    It must be set this before attaching a device to an IOAS/HWPT,
> 
> remove 'this'

Ack.

> > otherwise
> > + *    this option will be not effective on that IOAS/HWPT. User can 
> 
> Do we want to explicitly check this instead of leaving it no effect
> silently?

So, the idea here is:
If this option is unset, use the default SW_MSI from the driver
If this option is set, use it over the default SW_MSI from the driver

That's what the following statement "User can choose to let.." means.

> > choose to
> > + *    let kernel pick a base address, by simply ignoring this option or setting
> > + *    a value 0 to IOMMU_OPTION_SW_MSI_SIZE. Global option, object_id
> > must be 0
> > + * @IOMMU_OPTION_SW_MSI_SIZE:
> > + *    Change the size of the IOMMU mapping region for MSI doorbell(s). It
> > must
> > + *    be set this before attaching a device to an IOAS/HWPT, otherwise it
> > won't
> > + *    be effective on that IOAS/HWPT. The value is in MB, and the minimum
> > value
> > + *    is 1 MB. A value 0 (default) will invalidate the MSI doorbell base address
> > + *    value set to IOMMU_OPTION_SW_MSI_START. Global option, object_id
> > must be 0
> 
> hmm there is no check on the minimal value and enable the effect
> of value 0 in this patch.

Well, it's somewhat enforced by __aligned_u64 since it can't be any
value between 0 (disable) and 1 (minimal)?

And the override code checks "ctx->sw_msi_size".

> >  iommufd_device_attach_reserved_iova(struct iommufd_device *idev,
> >  				    struct iommufd_hwpt_paging
> > *hwpt_paging)
> >  {
> > +	struct iommufd_ctx *ictx = idev->ictx;
> >  	int rc;
> > 
> >  	lockdep_assert_held(&idev->igroup->lock);
> > 
> > +	/* Override it with a user-programmed SW_MSI region */
> > +	if (ictx->sw_msi_size && ictx->sw_msi_start != PHYS_ADDR_MAX)
> > +		idev->igroup->sw_msi_start = ictx->sw_msi_start;
> >  	rc = iopt_table_enforce_dev_resv_regions(&hwpt_paging->ioas->iopt,
> >  						 idev->dev,
> >  						 &idev->igroup-
> > >sw_msi_start);
> 
> what about moving above additions into 
> iopt_table_enforce_dev_resv_regions() which is all about finding
> a sw_msi address and can check the user setting internally?

We could. Probably would be cleaner by doing that in one place.

> > diff --git a/drivers/iommu/iommufd/io_pagetable.c
> > b/drivers/iommu/iommufd/io_pagetable.c
> > index 8a790e597e12..5d7f5ca1eecf 100644
> > --- a/drivers/iommu/iommufd/io_pagetable.c
> > +++ b/drivers/iommu/iommufd/io_pagetable.c
> > @@ -1446,7 +1446,9 @@ int iopt_table_enforce_dev_resv_regions(struct
> > io_pagetable *iopt,
> >  		if (sw_msi_start && resv->type == IOMMU_RESV_MSI)
> >  			num_hw_msi++;
> >  		if (sw_msi_start && resv->type == IOMMU_RESV_SW_MSI) {
> > -			*sw_msi_start = resv->start;
> > +			/* Bypass the driver-defined SW_MSI region, if preset
> > */
> > +			if (*sw_msi_start == PHYS_ADDR_MAX)
> > +				*sw_msi_start = resv->start;
> 
> the code is not about bypass. Instead it's to use the driver-defined
> region if user doesn't set it.

Ack:
			/* If being unset, Use the default IOMMU_RESV_SW_MSI */

Thanks
Nicolin