Re: [PATCH kernel 0/3 REPOST] vfio-pci: Add support for mmapping MSI-X table

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 24/06/17 01:17, Alex Williamson wrote:
> On Fri, 23 Jun 2017 15:06:37 +1000
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
>> On 23/06/17 07:11, Alex Williamson wrote:
>>> On Thu, 15 Jun 2017 15:48:42 +1000
>>> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>>   
>>>> Here is a patchset which Yongji was working on before
>>>> leaving IBM LTC. Since we still want to have this functionality
>>>> in the kernel (DPDK is the first user), here is a rebase
>>>> on the current upstream.
>>>>
>>>>
>>>> Current vfio-pci implementation disallows to mmap the page
>>>> containing MSI-X table in case that users can write directly
>>>> to MSI-X table and generate an incorrect MSIs.
>>>>
>>>> However, this will cause some performance issue when there
>>>> are some critical device registers in the same page as the
>>>> MSI-X table. We have to handle the mmio access to these
>>>> registers in QEMU emulation rather than in guest.
>>>>
>>>> To solve this issue, this series allows to expose MSI-X table
>>>> to userspace when hardware enables the capability of interrupt
>>>> remapping which can ensure that a given PCI device can only
>>>> shoot the MSIs assigned for it. And we introduce a new bus_flags
>>>> PCI_BUS_FLAGS_MSI_REMAP to test this capability on PCI side
>>>> for different archs.
>>>>
>>>> The patch 3 are based on the proposed patchset[1].
>>>>
>>>> Changelog
>>>> v3:
>>>> - rebased on the current upstream  
>>>
>>> There's something not forthcoming here, the last version I see from
>>> Yongji is this one:
>>>
>>> https://lists.linuxfoundation.org/pipermail/iommu/2016-June/017245.html
>>>
>>> Which was a 6-patch series where patches 2-4 tried to apply
>>> PCI_BUS_FLAGS_MSI_REMAP for cases that supported other platforms.  That
>>> doesn't exist here, so it's not simply a rebase.  Patch 1/ seems to
>>> equate this new flag to the IOMMU capability IOMMU_CAP_INTR_REMAP, but
>>> nothing is done here to match them together.  That patch also mentions
>>> the work Eric has done for similar features on ARM, but again those
>>> patches are dropped.  It seems like an incomplete feature now.  Thanks,  
>>
>>
>> Thanks! I suspected this is not the latest but could not find anything
>> better than we use internally for tests, and I could not reach Yongji for
>> comments whether this was the latest update.
>>
>> As I am reading the patches, I notice that the "msi remap" term is used all
>> over the place. While this remapping capability may be the case for x86/arm
>> (and therefore the IOMMU_CAP_INTR_REMAP flag makes sense), powernv does not
>> do remapping but provides hardware isolation. When we are allowing MSIX BAR
>> mapping to the userspace - the isolation is what we really care about. Will
>> it make sense to rename PCI_BUS_FLAGS_MSI_REMAP to
>> PCI_BUS_FLAGS_MSI_ISOLATED ?
> 
> I don't have a strong opinion either way, so long as it's fully
> described what the flag indicates.
> 
>> Another thing - the patchset enables PCI_BUS_FLAGS_MSI_REMAP when IOMMU
>> just advertises IOMMU_CAP_INTR_REMAP, not necessarily uses it, should the
>> patchset actually look at something like irq_remapping_enabled in
>> drivers/iommu/amd_iommu.c instead?
> 
> Interrupt remapping being enabled is implicit in IOMMU_CAP_INTR_REMAP,
> neither intel or amd iommu export the capability unless enabled.
> Nobody cares if it's supported but not enabled.  Thanks,


As I am reading the current drivers/vfio/vfio_iommu_type1.c, it feels like
MSIX BAR mappings can always be allowed for the type1 IOMMU as
vfio_iommu_type1_attach_group() performs this check:

msi_remap = resv_msi ? irq_domain_check_msi_remap() :
    iommu_capable(bus, IOMMU_CAP_INTR_REMAP);

and simply does not proceed if MSI remap is not supported. Is that correct
or I miss something here? Thanks.




-- 
Alexey