[dpdk-dev] [PATCH v3] vfio: fix workaround of BAR0 mapping

[Burakov, Anatoly]

On 13-Jul-18 11:11 AM, Takeshi Yoshimura wrote:
> The workaround of BAR0 mapping gives up and immediately returns an
> error if it cannot map around the MSI-X. However, recent version
> of VFIO allows MSIX mapping (*).
> 
> I fixed not to return immediately but try mapping. In old Linux, mmap
> just fails and returns the same error as the code before my fix . In
> recent Linux, mmap succeeds and this patch enables running DPDK in
> specific environments (e.g., ppc64le with HGST NVMe)
> 
> (*): "vfio-pci: Allow mapping MSIX BAR",
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
> commit/id=a32295c612c57990d17fb0f41e7134394b2f35f6
> 
> Fixes: 90a1633b2347 ("eal/linux: allow to map BARs with MSI-X tables")
> 
> Signed-off-by: Takeshi Yoshimura <t.yoshimura8869 at gmail.com>
> ---
> 
> Thanks, Anatoly.
> 
> I updated the patch not to affect behaviors of older Linux and
> other environments as well as possible. This patch adds another
> chance to mmap BAR0.
> 
> I noticed that the check at line 350 already includes the check
> of page size, so this patch does not fix the check.
> 
> Regards,
> Takeshi

Hi Takeshi,

Please correct me if i'm wrong, but i'm not sure the old behavior is kept.

Let's say we're running an old kernel, which doesn't allow mapping MSI-X 
BARs. If MSI-X starts at beginning of the BAR (floor-aligned to page 
size), and ends at or beyond end of BAR (ceiling-aligned to page size). 
In that situation, old code just skipped the BAR and returned 0.

We then exited the function, and there's a check for return value right 
after pci_vfio_mmap_bar() that stop continuing if we fail to map 
something. In the old code, we would continue as we went, and finish the 
rest of our mappings. With your new code, you're attempting to map the 
BAR, it fails, and you will return -1 on older kernels.

I believe what we really need here is the following:

1) If this is a BAR containing MSI-X vector, first try mapping the 
entire BAR. If it succeeds, great - that would be your new kernel behavior.
2) If we failed on step 1), check to see if we can map around the BAR. 
If we can, try to map around it like the current code does. If we cannot 
map around it (i.e. if MSI-X vector, page aligned, occupies entire BAR), 
then we simply return 0 and skip the BAR.

That, i would think, would keep the old behavior and enable the new one.

Does that make sense?

-- 
Thanks,
Anatoly