Re: [PATCH v3] vfio: fix workaround of BAR0 mapping

[Takeshi Yoshimura <t.yoshimura8869@gmail.com>]

2018-07-13 20:08 GMT+09:00 Burakov, Anatoly <anatoly.burakov@intel.com>:
> On 13-Jul-18 12:00 PM, Burakov, Anatoly wrote:
>>
>> 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@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?
>>
>
> I envision this to look something like this:
>
> bool again = false;
> do {
>         if (again) {
>                 // set up mmap-around
>                 if (cannot map around)
>                         return 0;
>         }
>         // try mapping
>         if (map_failed && msix_table->bar_index == bar_index) {
>                 again = true;
>                 continue;
>         }
>         if (map_failed)
>                 return -1;
>         break/return 0;
> } while (again);
>
> --
> Thanks,
> Anatoly

That makes sense. The return code was not same as old one in some paths.

I wrote a code based on your idea. It works at least in my ppc64 and
x86 machines, but I am concerned that the error messages for
pci_map_resource() confuse users in old Linux. I saw a message like
this (even if I could mmap):
EAL: pci_map_resource(): cannot mmap(15, 0x728ee3a30000, 0x4000, 0x0):
Invalid argument (0xffffffffffffffff)

But anyway, I send it in the next email, and please check if there is
any other problems in the code.

Thanks,
Takeshi