linux-kernel - Re: [PATCH 06/16] iommu/dma: use page allocation function provided by iommu-pages.h

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Message-ID: <CA+CK2bDyKP6RT1ZWLKXBcRrfSuONo9LPicskc9+ek=FX_KhwYg@mail.gmail.com>
Date:   Tue, 28 Nov 2023 18:06:34 -0500
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Robin Murphy <robin.murphy@....com>
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Subject: Re: [PATCH 06/16] iommu/dma: use page allocation function provided by iommu-pages.h

On Tue, Nov 28, 2023 at 5:59 PM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2023-11-28 10:50 pm, Pasha Tatashin wrote:
> > On Tue, Nov 28, 2023 at 5:34 PM Robin Murphy <robin.murphy@....com> wrote:
> >>
> >> On 2023-11-28 8:49 pm, Pasha Tatashin wrote:
> >>> Convert iommu/dma-iommu.c to use the new page allocation functions
> >>> provided in iommu-pages.h.
> >>
> >> These have nothing to do with IOMMU pagetables, they are DMA buffers and
> >> they belong to whoever called the corresponding dma_alloc_* function.
> >
> > Hi Robin,
> >
> > This is true, however, we want to account and observe the pages
> > allocated by IOMMU subsystem for DMA buffers, as they are essentially
> > unmovable locked pages. Should we separate IOMMU memory from KVM
> > memory all together and add another field to /proc/meminfo, something
> > like "iommu -> iommu pagetable and dma memory", or do we want to
> > export DMA memory separately from IOMMU page tables?
>
> These are not allocated by "the IOMMU subsystem", they are allocated by
> the DMA API. Even if you want to claim that a driver pinning memory via
> iommu_dma_ops is somehow different from the same driver pinning the same
> amount of memory via dma-direct when iommu.passthrough=1, it's still
> nonsense because you're failing to account the pages which iommu_dma_ops
> gets from CMA, dma_common_alloc_pages(), dynamic SWIOTLB, the various
> pools, and so on.
>
> Thanks,
> Robin.
>
> > Since, I included DMA memory, I specifically removed mentioning of
> > IOMMU page tables in the most of places, and only report it as IOMMU
> > memory. However, since it is still bundled together with SecPageTables
> > it can be confusing.
> >
> > Pasha