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

[Jason Gunthorpe]

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.

Jason