On 2021-06-07 03:42, chenxiang wrote:
From: Xiang Chen chenxiang66@hisilicon.com
When rmmod the driver of the last device in the group, cached iovas are not used, and it is better to free them to save memories. And also export function free_rcache_cached_iovas() and iommu_domain_to_iova().
How common is it to use a device a significant amount, then unbind its driver and not use it for long enough to care about? Off-hand I can't think of a particularly realistic use-case which looks like that - the closest I can imagine is unbinding a default kernel driver to replace it with VFIO, but I would expect the set of devices intended for assignment to be distinct from the set of devices used by the host itself, and thus the host driver wouldn't have actually done much to generate a lot of DMA mappings in that initial period. Is my expectation there wrong?
If there is such a case, how much memory does this actually save in practice? The theoretical worst-case should be roughly 4 * 128 * 6 * sizeof(struct iova) bytes per CPU, which is around 192KB assuming 64-byte kmem cache alignment. However it seems rather unlikely in practice to have every possible cache entry of every size used, so if saving smaller amounts of memory is a concern wouldn't you also want to explicitly drain the flush queues (16KB per CPU) and maybe look at trying to reclaim the unused pagetable pages from the domain itself - that ~192KB worth of cached IOVAs represents ~32K pages worth of IOVA space, which for an implementation like io-pgtable-arm with the 4KB granule means ~256KB worth of non-leaf pagetables left behind.
I'm not against the idea of having a mechanism to "compact" an idle DMA domain if there are convincing numbers to back it up, but the actual implementation would need to be better than this as well - having the IOMMU core code poking directly into the internals of the iommu-dma abstraction is not the way to go, and exporting anything to modules, which the IOMU core is not, smells suspicious.
Robin.
Xiang Chen (4): iommu/iova: add a function to free all rcached iovas and export it iommu/iova: use function free_rcache_cached_iovas() to free all rcached iovas dma-iommu: add a interface to get iova_domain from iommu domain iommu: free cached iovas when rmmod the driver of the last device in the group
drivers/iommu/dma-iommu.c | 7 +++++++ drivers/iommu/iommu.c | 7 +++++++ drivers/iommu/iova.c | 17 ++++++++++++----- include/linux/dma-iommu.h | 6 ++++++ include/linux/iova.h | 5 +++++ 5 files changed, 37 insertions(+), 5 deletions(-)