On 2021-06-07 03:42, chenxiang wrote:
From: Xiang Chen <[email protected]>
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(-)
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