Re: [PATCH v5 05/16] swiotlb: Add restricted DMA pool initialization

[Claire Chang <tientzu@chromium.org>]

On Fri, Apr 23, 2021 at 7:34 PM Steven Price <steven.price@arm.com> wrote:
>
> On 22/04/2021 09:14, Claire Chang wrote:
> > Add the initialization function to create restricted DMA pools from
> > matching reserved-memory nodes.
> >
> > Signed-off-by: Claire Chang <tientzu@chromium.org>
> > ---
> >   include/linux/device.h  |  4 +++
> >   include/linux/swiotlb.h |  3 +-
> >   kernel/dma/swiotlb.c    | 80 +++++++++++++++++++++++++++++++++++++++++
> >   3 files changed, 86 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/device.h b/include/linux/device.h
> > index 38a2071cf776..4987608ea4ff 100644
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -416,6 +416,7 @@ struct dev_links_info {
> >    * @dma_pools:      Dma pools (if dma'ble device).
> >    * @dma_mem:        Internal for coherent mem override.
> >    * @cma_area:       Contiguous memory area for dma allocations
> > + * @dma_io_tlb_mem: Internal for swiotlb io_tlb_mem override.
> >    * @archdata:       For arch-specific additions.
> >    * @of_node:        Associated device tree node.
> >    * @fwnode: Associated device node supplied by platform firmware.
> > @@ -521,6 +522,9 @@ struct device {
> >   #ifdef CONFIG_DMA_CMA
> >       struct cma *cma_area;           /* contiguous memory area for dma
> >                                          allocations */
> > +#endif
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +     struct io_tlb_mem *dma_io_tlb_mem;
> >   #endif
> >       /* arch specific additions */
> >       struct dev_archdata     archdata;
> > diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> > index 216854a5e513..03ad6e3b4056 100644
> > --- a/include/linux/swiotlb.h
> > +++ b/include/linux/swiotlb.h
> > @@ -72,7 +72,8 @@ extern enum swiotlb_force swiotlb_force;
> >    *          range check to see if the memory was in fact allocated by this
> >    *          API.
> >    * @nslabs: The number of IO TLB blocks (in groups of 64) between @start and
> > - *           @end. This is command line adjustable via setup_io_tlb_npages.
> > + *           @end. For default swiotlb, this is command line adjustable via
> > + *           setup_io_tlb_npages.
> >    * @used:   The number of used IO TLB block.
> >    * @list:   The free list describing the number of free entries available
> >    *          from each index.
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index 57a9adb920bf..ffbb8724e06c 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -39,6 +39,13 @@
> >   #ifdef CONFIG_DEBUG_FS
> >   #include <linux/debugfs.h>
> >   #endif
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +#include <linux/io.h>
> > +#include <linux/of.h>
> > +#include <linux/of_fdt.h>
> > +#include <linux/of_reserved_mem.h>
> > +#include <linux/slab.h>
> > +#endif
> >
> >   #include <asm/io.h>
> >   #include <asm/dma.h>
> > @@ -681,3 +688,76 @@ static int __init swiotlb_create_default_debugfs(void)
> >   late_initcall(swiotlb_create_default_debugfs);
> >
> >   #endif
> > +
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
> > +                                 struct device *dev)
> > +{
> > +     struct io_tlb_mem *mem = rmem->priv;
> > +     unsigned long nslabs = rmem->size >> IO_TLB_SHIFT;
> > +
> > +     if (dev->dma_io_tlb_mem)
> > +             return 0;
> > +
> > +     /* Since multiple devices can share the same pool, the private data,
> > +      * io_tlb_mem struct, will be initialized by the first device attached
> > +      * to it.
> > +      */
> > +     if (!mem) {
> > +             mem = kzalloc(struct_size(mem, slots, nslabs), GFP_KERNEL);
> > +             if (!mem)
> > +                     return -ENOMEM;
> > +#ifdef CONFIG_ARM
> > +             if (!PageHighMem(pfn_to_page(PHYS_PFN(rmem->base)))) {
> > +                     kfree(mem);
> > +                     return -EINVAL;
> > +             }
> > +#endif /* CONFIG_ARM */
> > +             swiotlb_init_io_tlb_mem(mem, rmem->base, nslabs, false);
> > +
> > +             rmem->priv = mem;
> > +     }
> > +
> > +#ifdef CONFIG_DEBUG_FS
> > +     if (!io_tlb_default_mem->debugfs)
> > +             io_tlb_default_mem->debugfs =
> > +                     debugfs_create_dir("swiotlb", NULL);
>
> At this point it's possible for io_tlb_default_mem to be NULL, leading
> to a splat.

Thanks for pointing this out.

>
> But even then if it's not and we have the situation where debugfs==NULL
> then the debugfs_create_dir() here will cause a subsequent attempt in
> swiotlb_create_debugfs() to fail (directory already exists) leading to
> mem->debugfs being assigned an error value. I suspect the creation of
> the debugfs directory needs to be separated from io_tlb_default_mem
> being set.

debugfs creation should move into the if (!mem) {...} above to avoid
duplication.
I think having a separated struct dentry pointer for the default
debugfs should be enough?

if (!debugfs)
    debugfs = debugfs_create_dir("swiotlb", NULL);
swiotlb_create_debugfs(mem, rmem->name, debugfs);

>
> Other than that I gave this series a go with our prototype of Arm's
> Confidential Computer Architecture[1] - since the majority of the
> guest's memory is protected from the host the restricted DMA pool allows
> (only) a small area to be shared with the host.
>
> After fixing (well hacking round) the above it all seems to be working
> fine with virtio drivers.
>
> Thanks,
>
> Steve
>
> [1]
> https://www.arm.com/why-arm/architecture/security-features/arm-confidential-compute-architecture