Re: [PATCH v3 2/7] iommu/riscv: Add RISC-V IOMMU platform device driver

[Baolu Lu]

On 5/1/24 10:20 PM, Jason Gunthorpe wrote:
> On Wed, May 01, 2024 at 06:26:20PM +0800, Baolu Lu wrote:
> > On 2024/5/1 4:01, Tomasz Jeznach wrote:
> > > +static int riscv_iommu_init_check(struct riscv_iommu_device *iommu)
> > > +{
> > > +	u64 ddtp;
> > > +
> > > +	/*
> > > +	 * Make sure the IOMMU is switched off or in pass-through mode during regular
> > > +	 * boot flow and disable translation when we boot into a kexec kernel and the
> > > +	 * previous kernel left them enabled.
> > > +	 */
> > > +	ddtp = riscv_iommu_readq(iommu, RISCV_IOMMU_REG_DDTP);
> > > +	if (ddtp & RISCV_IOMMU_DDTP_BUSY)
> > > +		return -EBUSY;
> > > +
> > > +	if (FIELD_GET(RISCV_IOMMU_DDTP_MODE, ddtp) > RISCV_IOMMU_DDTP_MODE_BARE) {
> > > +		if (!is_kdump_kernel())
> > Is kdump supported for RISC-V architectures?  If so, the documentation
> > in Documentation/admin-guide/kdump/kdump.rst might need an update.
> >
> > There is a possibility of ongoing DMAs during the boot process of the
> > kdump capture kernel because there's a small chance of legacy DMA setups
> > targeting any memory location. Kdump typically allows these ongoing DMA
> > transfers to complete, assuming they were intended for valid memory
> > regions.
> >
> > The IOMMU subsystem implements a default domain deferred attachment
> > mechanism for this. In the kdump capture kernel, the whole device
> > context tables are copied from the original kernel and will be
> > overridden once the device driver calls the kernel DMA interface for the
> > first time. This assumes that all old DMA transfers are completed after
> > the driver's takeover.
> >
> > Will you consider this for RISC-V architecture as well?
> It seems we decided not to do that mess in ARM..
>
> New architectures doing kdump should put the iommu in a full blocking
> state before handing over the next kernel, and this implies that
> devices drivers need to cleanly suspend their DMAs before going into
> the next kernel.

Glad to hear that. :-)

With the above consideration, the driver should consider it an error
case where the iommu is not in the blocking state, and it's in the kdump
kernel, right?

If so, probably the iommu driver should always return failure when the
iommu is not in the blocking state. However, the RISC-V's logic is:

 - if this is a kdump kernel, just disable iommu;
 - otherwise, failure case.

This logic seems problematic.

Best regards,
baolu