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

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Date: Wed, 1 May 2024 11:20:37 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Tomasz Jeznach <tjeznach@...osinc.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Anup Patel <apatel@...tanamicro.com>,
	Sunil V L <sunilvl@...tanamicro.com>,
	Nick Kossifidis <mick@....forth.gr>,
	Sebastien Boeuf <seb@...osinc.com>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	iommu@...ts.linux.dev, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux@...osinc.com
Subject: Re: [PATCH v3 2/7] iommu/riscv: Add RISC-V IOMMU platform device
 driver

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