Re: [PATCH v13 03/15] iommu/dma: Allow MSI-only cookies

Robin Murphy
2016-10-10T14:26:51Z

Hi Alex, Eric,

On 06/10/16 21:17, Alex Williamson wrote:
> On Thu, 6 Oct 2016 08:45:19 +0000
> Eric Auger <eric.auger@redhat.com> wrote:
>
>> From: Robin Murphy <robin.murphy@arm.com>
>>
>> IOMMU domain users such as VFIO face a similar problem to DMA API ops
>> with regard to mapping MSI messages in systems where the MSI write is
>> subject to IOMMU translation. With the relevant infrastructure now in
>> place for managed DMA domains, it's actually really simple for other
>> users to piggyback off that and reap the benefits without giving up
>> their own IOVA management, and without having to reinvent their own
>> wheel in the MSI layer.
>>
>> Allow such users to opt into automatic MSI remapping by dedicating a
>> region of their IOVA space to a managed cookie.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>
>> ---
>>
>> v1 -> v2:
>> - compared to Robin's version
>> - add NULL last param to iommu_dma_init_domain
>> - set the msi_geometry aperture
>> - I removed
>> if (base < U64_MAX - size)
>> reserve_iova(iovad, iova_pfn(iovad, base + size), ULONG_MAX);
>> don't get why we would reserve something out of the scope of the iova domain?
>> what do I miss?
>> ---
>> drivers/iommu/dma-iommu.c | 40 ++++++++++++++++++++++++++++++++++++++++
>> include/linux/dma-iommu.h | 9 +++++++++
>> 2 files changed, 49 insertions(+)
>>
>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>> index c5ab866..11da1a0 100644
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> @@ -716,3 +716,43 @@ void iommu_dma_map_msi_msg(int irq, struct msi_msg *msg)
>> msg->address_lo += lower_32_bits(msi_page->iova);
>> }
>> }
>> +
>> +/**
>> + * iommu_get_dma_msi_region_cookie - Configure a domain for MSI remapping only
>
> Should this perhaps be iommu_setup_dma_msi_region_cookie, or something
> along those lines. I'm not sure what we're get'ing. Thanks,

What we're getting is private third-party resources for the iommu_domain
given in the argument. It's a get/put rather than alloc/free model since
we operate opaquely on the domain as a container, rather than on the
actual resource in question (an IOVA allocator).

Since this particular use case is slightly different from the normal
flow and has special initialisation requirements, it seemed a lot
cleaner to simply combine that initialisation operation with the
prerequisite "get" into a single call. Especially as it helps emphasise
that this is not 'normal' DMA cookie usage.

>
> Alex
>
>> + * @domain: IOMMU domain to prepare
>> + * @base: Base address of IOVA region to use as the MSI remapping aperture
>> + * @size: Size of the desired MSI aperture
>> + *
>> + * Users who manage their own IOVA allocation and do not want DMA API support,
>> + * but would still like to take advantage of automatic MSI remapping, can use
>> + * this to initialise their own domain appropriately.
>> + */
>> +int iommu_get_dma_msi_region_cookie(struct iommu_domain *domain,
>> + dma_addr_t base, u64 size)
>> +{
>> + struct iommu_dma_cookie *cookie;
>> + struct iova_domain *iovad;
>> + int ret;
>> +
>> + if (domain->type == IOMMU_DOMAIN_DMA)
>> + return -EINVAL;
>> +
>> + ret = iommu_get_dma_cookie(domain);
>> + if (ret)
>> + return ret;
>> +
>> + ret = iommu_dma_init_domain(domain, base, size, NULL);
>> + if (ret) {
>> + iommu_put_dma_cookie(domain);
>> + return ret;
>> + }

It *is* necessary to explicitly reserve the upper part of the IOVA
domain here - the aforementioned "special initialisation" - because
dma_32bit_pfn is only an optimisation hint to prevent the allocator
walking down from the very top of the the tree every time when devices
with different DMA masks share a domain (I'm in two minds as to whether
to tweak the way the iommu-dma code uses it in this respect, now that I
fully understand things). The only actual upper limit to allocation is
the DMA mask passed into each alloc_iova() call, so if we want to ensure
IOVAs are really allocated within this specific region, we have to carve
out everything above it.

Robin.

>> +
>> + domain->msi_geometry.aperture_start = base;
>> + domain->msi_geometry.aperture_end = base + size - 1;
>> +
>> + cookie = domain->iova_cookie;
>> + iovad = &cookie->iovad;
>> +
>> + return 0;
>> +}
>> +EXPORT_SYMBOL(iommu_get_dma_msi_region_cookie);
>> diff --git a/include/linux/dma-iommu.h b/include/linux/dma-iommu.h
>> index 32c5890..1c55413 100644
>> --- a/include/linux/dma-iommu.h
>> +++ b/include/linux/dma-iommu.h
>> @@ -67,6 +67,9 @@ int iommu_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
>> /* The DMA API isn't _quite_ the whole story, though... */
>> void iommu_dma_map_msi_msg(int irq, struct msi_msg *msg);
>>
>> +int iommu_get_dma_msi_region_cookie(struct iommu_domain *domain,
>> + dma_addr_t base, u64 size);
>> +
>> #else
>>
>> struct iommu_domain;
>> @@ -90,6 +93,12 @@ static inline void iommu_dma_map_msi_msg(int irq, struct msi_msg *msg)
>> {
>> }
>>
>> +static inline int iommu_get_dma_msi_region_cookie(struct iommu_domain *domain,
>> + dma_addr_t base, u64 size)
>> +{
>> + return -ENODEV;
>> +}
>> +
>> #endif /* CONFIG_IOMMU_DMA */
>> #endif /* __KERNEL__ */
>> #endif /* __DMA_IOMMU_H */
>