Re: [dpdk-dev] [PATCH] vfio: fix expanding DMA area in ppc64le

[Burakov, Anatoly]

On 28-Jun-19 2:47 PM, Burakov, Anatoly wrote:
On 28-Jun-19 12:38 PM, Takeshi T Yoshimura wrote:
To: "Mo, YufengX" <yufengx...@intel.com>, dev@dpdk.org
From: "Burakov, Anatoly" <anatoly.bura...@intel.com>
Date: 06/26/2019 06:43PM
Cc: d...@ibm.com, prad...@us.ibm.com, Takeshi Yoshimura
<t...@jp.ibm.com>
Subject: [EXTERNAL] Re: [dpdk-dev] [PATCH] vfio: fix expanding DMA
area in ppc64le
On 18-Jun-19 3:37 AM, Mo, YufengX wrote:
From: Takeshi Yoshimura <t...@jp.ibm.com>

In ppc64le, expanding DMA areas always fail because we cannot
remove
a DMA window. As a result, we cannot allocate more than one memseg
in
ppc64le. This is because vfio_spapr_dma_mem_map() doesn't unmap all
the mapped DMA before removing the window. This patch fixes this
incorrect behavior.

I added a global variable to track current window size since we do
not have better ways to get exact size of it than doing so. sPAPR
IOMMU seems not to provide any ways to get window size with ioctl
interfaces. rte_memseg_walk*() is currently used to calculate
window
size, but it walks memsegs that are marked as used, not mapped. So,
we need to determine if a given memseg is mapped or not, otherwise
the ioctl reports errors due to attempting to unregister memory
addresses that are not registered. The global variable is excluded
in non-ppc64le binaries.

Similar problems happen in user maps. We need to avoid attempting
to
unmap the address that is given as the function's parameter. The
compaction of user maps prevents us from passing correct length for
unmapping DMA at the window recreation. So, I removed it in
ppc64le.

I also fixed the order of ioctl for unregister and unmap. The ioctl
for unregister sometimes report device busy errors due to the
existence of mapped area.

Signed-off-by: Takeshi Yoshimura <t...@jp.ibm.com>
---

OK there are three patches, and two v1's with two different authors
in
reply to the same original patch. There's too much going on here, i
can't review this. Needs splitting.

Also, #ifdef-ing out the map merging seems highly suspect.

With regards to "walking used memsegs, not mapped", unless i'm
misunderstanding something, these are the same - whenever a segment
is
mapped, it is marked as used, and whenever it is unmapped, it is
marked
as free. Could you please explain what is the difference and why is
this
needed?

Is the point of contention here being the fact that whenever the
unmap
callback arrives, the segments still appear used when iterating over
the
map? If that's the case, then i think it would be OK to mark them as
unused *before* triggering callbacks, and chances are some of this
code
wouldn't be needed. That would require a deprecation notice though,
because the API behavior will change (even if this fact is not
documented properly).

--
Thanks,
Anatoly



I am the author of this patch. We should ignore a patch from YufengX Mo.

From my code reading, a memsg is at first marked as used when it is 
allocated. Then, the memseg is passed to vfio_spapr_dma_mem_map(). 
The callback iterates all the used (i.e., allocated) memsegs and call 
ioctl for mapping VA to IOVA. So, when vfio_spapr_dma_mem_map() is 
called, passed memsegs can be non-mapped but marked as used. As a 
result, an attempt to unmap non-mapped area happens during DMA window 
expansion. This is the difference and why this fix was needed.

i think it would be OK to mark them as unused *before* triggering 
callbacks

Yes, my first idea was the same as yours, but I was also worried that 
it might cause inconsistent API behavior as you also pointed out. If 
you think so, I think I can rewrite the patch without ugly #ifdef.

Unfortunately, I don't have enough time for writing code next week and 
next next week. So, I will resubmit the revised patch weeks later.

I think the approach with fixing the mem callbacks to report the 
unmapped segments as no longer used would be better.

As far as i can remember at the point where callbacks are triggered, the 
memory is already removed from malloc heap and from all processes. Each 
secondary also stores their own shadow copy of the memory map, so 
removing the "used" flags from the main map will not have any 
consequences as far as correctness is concerned. Each callback is also 
getting the memory area being removed as parameters, so if there is code 
that needs to be run taking into account that memory area, it can be done.

Existing code may rely on this behavior (even though it doesn't make 
much sense now that i think of it), so going with this approach *will* 
require a deprecation notice and can only be done in the next release.


One *other* way to fix this would be to stored the pages being removed 
in a struct, and pass it as parameters to the _walk() window size 
calculation function. This would avoid needless API changes and handle 
this case correctly.




Regards,
Takeshi






--
Thanks,
Anatoly