linux-kernel - Re: [PATCH v1 07/17] dma-mapping: Implement link/unlink ranges API

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Message-ID: <20241104121924.GC35848@ziepe.ca>
Date: Mon, 4 Nov 2024 08:19:24 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christoph Hellwig <hch@....de>
Cc: Robin Murphy <robin.murphy@....com>, Leon Romanovsky <leon@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Leon Romanovsky <leonro@...dia.com>,
	Keith Busch <kbusch@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 07/17] dma-mapping: Implement link/unlink ranges API

On Mon, Nov 04, 2024 at 10:10:48AM +0100, Christoph Hellwig wrote:
> >> +		arch_sync_dma_for_device(phys, size, dir);
> >
> > Plus if the aim is to pass P2P and whatever arbitrary physical addresses 
> > through here as well, how can we be sure this isn't going to explode?
> 
> That's a good point.  Only mapped through host bridge P2P can even
> end up here, so the address is a perfectly valid physical address
> in the host.  But I'm not sure if all arch_sync_dma_for_device
> implementations handle IOMMU memory fine.

I was told on x86 if you do a cache flush operation on MMIO there is a
chance it will MCE. Recently had some similar discussions about ARM
where it was asserted some platforms may have similar.

It would be safest to only call arch flushing calls on memory that is
mapped cachable. We can assume that a P2P target is never CPU
mapped cachable, regardless of how the DMA is routed.

Jason