Re: [Qemu-ppc] [Qemu-devel] [PATCH qemu v3] RFC: vfio-pci: Allow mmap of MSIX BAR

[Auger Eric]

Hi Alex,
On 24/01/18 16:02, Alex Williamson wrote:
> On Wed, 24 Jan 2018 15:13:06 +0100
> Auger Eric <address@hidden> wrote:
> 
>> Hi Alex,
>> On 23/01/18 20:55, Alex Williamson wrote:
>>> On Tue, 23 Jan 2018 16:34:34 +0100
>>> Auger Eric <address@hidden> wrote:
>>>   
>>>> Hi Alexey,
>>>> On 23/01/18 02:22, Alexey Kardashevskiy wrote:  
>>>>> This makes use of a new VFIO_REGION_INFO_CAP_MSIX_MAPPABLE capability
>>>>> which tells that a region with MSIX data can be mapped entirely, i.e.
>>>>> the VFIO PCI driver won't prevent MSIX vectors area from being mapped.
>>>>>
>>>>> With this change, all BARs are mapped in a single chunk and MSIX vectors
>>>>> are emulated on top unless the machine requests not to by defining and
>>>>> enabling a new "vfio-no-msix-emulation" property. At the moment only
>>>>> sPAPR machine does so - it prohibits MSIX emulation and does not allow
>>>>> enabling it as it does not define the "set" callback for the new property;
>>>>> the new property also does not appear in "-machine pseries,help".
>>>>>
>>>>> If MSIX vectors section is not aligned to the page size, the KVM memory
>>>>> listener does not register it with the KVM as a memory slot and MSIX is
>>>>> emulated by QEMU as before. This may create MMIO RAM memory sections with
>>>>> an address or/and a size not aligned which will make vfio_dma_map() fail;
>>>>> to address this, this makes treats such failures as non-fatal.
>>>>>
>>>>> This requires the kernel change - "vfio-pci: Allow mapping MSIX BAR" -
>>>>> for the new capability: https://www.spinics.net/lists/kvm/msg160282.html
>>>>>
>>>>> Signed-off-by: Alexey Kardashevskiy <address@hidden>
>>>>> ---
>>>>> Changes:
>>>>> v3:
>>>>> * vfio_listener_region_add() won't make qemu exit if failed on MMIO MR
>>>>>
>>>>> ---
>>>>>  include/hw/vfio/vfio-common.h |  1 +
>>>>>  linux-headers/linux/vfio.h    |  5 +++++
>>>>>  hw/ppc/spapr.c                |  7 +++++++
>>>>>  hw/vfio/common.c              | 19 +++++++++++++++++++
>>>>>  hw/vfio/pci.c                 | 10 ++++++++++
>>>>>  5 files changed, 42 insertions(+)
>>>>>
>>>>> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
>>>>> index f3a2ac9..927d600 100644
>>>>> --- a/include/hw/vfio/vfio-common.h
>>>>> +++ b/include/hw/vfio/vfio-common.h
>>>>> @@ -171,6 +171,7 @@ int vfio_get_region_info(VFIODevice *vbasedev, int 
>>>>> index,
>>>>>                           struct vfio_region_info **info);
>>>>>  int vfio_get_dev_region_info(VFIODevice *vbasedev, uint32_t type,
>>>>>                               uint32_t subtype, struct vfio_region_info 
>>>>> **info);
>>>>> +bool vfio_is_cap_present(VFIODevice *vbasedev, uint16_t cap_type, int 
>>>>> region);
>>>>>  #endif
>>>>>  extern const MemoryListener vfio_prereg_listener;
>>>>>  
>>>>> diff --git a/linux-headers/linux/vfio.h b/linux-headers/linux/vfio.h
>>>>> index 4312e96..b45182e 100644
>>>>> --- a/linux-headers/linux/vfio.h
>>>>> +++ b/linux-headers/linux/vfio.h
>>>>> @@ -301,6 +301,11 @@ struct vfio_region_info_cap_type {
>>>>>  #define VFIO_REGION_SUBTYPE_INTEL_IGD_HOST_CFG   (2)
>>>>>  #define VFIO_REGION_SUBTYPE_INTEL_IGD_LPC_CFG    (3)
>>>>>  
>>>>> +/*
>>>>> + * The MSIX mappable capability informs that MSIX data of a BAR can be 
>>>>> mmapped.
>>>>> + */
>>>>> +#define VFIO_REGION_INFO_CAP_MSIX_MAPPABLE       3
>>>>> +
>>>>>  /**
>>>>>   * VFIO_DEVICE_GET_IRQ_INFO - _IOWR(VFIO_TYPE, VFIO_BASE + 9,
>>>>>   *                                   struct vfio_irq_info)
>>>>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>>>>> index d1acfe8..5ff43ce 100644
>>>>> --- a/hw/ppc/spapr.c
>>>>> +++ b/hw/ppc/spapr.c
>>>>> @@ -2789,6 +2789,11 @@ static void spapr_set_modern_hotplug_events(Object 
>>>>> *obj, bool value,
>>>>>      spapr->use_hotplug_event_source = value;
>>>>>  }
>>>>>  
>>>>> +static bool spapr_get_msix_emulation(Object *obj, Error **errp)
>>>>> +{
>>>>> +    return true;
>>>>> +}
>>>>> +
>>>>>  static char *spapr_get_resize_hpt(Object *obj, Error **errp)
>>>>>  {
>>>>>      sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
>>>>> @@ -2870,6 +2875,8 @@ static void spapr_instance_init(Object *obj)
>>>>>      object_property_set_description(obj, "vsmt",
>>>>>                                      "Virtual SMT: KVM behaves as if this 
>>>>> were"
>>>>>                                      " the host's SMT mode", 
>>>>> &error_abort);
>>>>> +    object_property_add_bool(obj, "vfio-no-msix-emulation",
>>>>> +                             spapr_get_msix_emulation, NULL, NULL);
>>>>>  }
>>>>>  
>>>>>  static void spapr_machine_finalizefn(Object *obj)
>>>>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
>>>>> index b77be3a..842c5b2 100644
>>>>> --- a/hw/vfio/common.c
>>>>> +++ b/hw/vfio/common.c
>>>>> @@ -530,6 +530,10 @@ static void vfio_listener_region_add(MemoryListener 
>>>>> *listener,
>>>>>      return;
>>>>>  
>>>>>  fail:
>>>>> +    if (memory_region_is_ram_device(section->mr)) {
>>>>> +        error_report("failed to vfio_dma_map. pci p2p may not work");
>>>>> +        return;
>>>>> +    }    
>>>>
>>>> I don't think this is an acceptable solution as it produces plenty of
>>>> errors such as
>>>>
>>>> qemu-system-aarch64: VFIO_MAP_DMA: -22
>>>> qemu-system-aarch64: vfio_dma_map(0x30a7ab90, 0x8000000000, 0x2000,
>>>> 0xfffd79a00000) = -22 (Invalid argument)  
>>>
>>> This much of the error above could be avoided by looking at the type1
>>> page size bitmap before trying to perform the mapping.
>>>   
>>>> qemu-system-aarch64: failed to vfio_dma_map. pci p2p may not work  
>>>
>>> This of course is still valid though we can continue to discuss if it's
>>> worthwhile user information.
>>>    
>>>> testing context is: aarch64 + Mellanox CX-4 PF where we still try to map
>>>> the BAR0 area [0 - 0x2000] before the MSI-X table which is not aligned
>>>> with the host 64kB page.
>>>>
>>>> Region 0: Memory at 80400000000 (64-bit, prefetchable) [size=32M]
>>>> Capabilities: [9c] MSI-X: Enable+ Count=64 Masked-
>>>>            Vector table: BAR=0 offset=00002000
>>>>            PBA: BAR=0 offset=00003000
>>>>
>>>>
>>>> To me a kind of mmaps region fixup looks required?  
>>>
>>> I don't think so.  vfio is now telling us that we can mmap the entire
>>> BAR, this is true.  The type1 info is telling us that the minimum DMA
>>> mapping granularity, PAGE_SIZE on the host (64K in your case).  The
>>> MSI-X offset on this device introduces an 8K range that type1 cannot
>>> map and QEMU already has all the information it needs to know this.  The
>>> solutions are either to move MSI-X elsewhere or to extend type1 to
>>> allow DMA mappings down to the IOMMU minimum granularity.
>>>
>>> The MSI-X fixup that is now bypassed avoided this not generating an
>>> mmap over the vector table, which meant this 8K region wasn't covered
>>> by a memory slot and therefore never got passed through region_add.  So
>>> p2p dma was silently disabled.  Now we do mmap the entire BAR, but we
>>> add the MSI-X region on top of it so that when it gets flattened we end
>>> up with these sub-page slots.  Skipping them at region_add has the same
>>> overall effect.
>>>
>>> Before:
>>>
>>>   |-| <-- MSI-X vector emulation MR
>>> +
>>>          |---PAGE_SIZE aligned mmap MR----|
>>> +
>>> |------------slow mapped BAR M------------|
>>>
>>> =
>>>
>>>          |----Resulting region_add--------| (PAGE_SIZE aligned)
>>>
>>> After:
>>>
>>>   |-| <-- MSI-X vector emulation MR
>>> +
>>> |------------PAGE_SIZE aligned mmap MR----|
>>> +
>>> |------------slow mapped BAR M------------|
>>>
>>> =
>>>
>>> |-| |----Resulting region_add-------------| (Gap no longer PAGE_SIZE 
>>> aligned)
>>>
>>> So doesn't it make sense that region_add would now filter these out?
>>> Should we continue to silently skip unmapped MMIO?  Thanks,  
>>
>> I am confused because I don't see the vfio_region_read/write for
>> segments whose mmap failed. Nevertheless the assignment looks to work
>> properly.
> 
> Segments whose mmap failed or whose dma map failed?
dma map sorry
  When we have the
> "after" case above, the ram_device mmap covers the entire direct access
> range to the device, therefore the flattened view will hit those
> MemoryRegion.ops first, which will be ram_device_mem_ops.
Ah OK this is the piece I was missing.

OK so above description makes sense to me. However at the moment the
user doesn't get a clear indication about what's wrong and this is what
I pointed out first.

Thanks

Eric

Thanks

Eric
  We'll only
> get to vfio_region_ops in the "before" case where the flattened map
> exposes the slow MR.  Thanks,
> 
> Alex
>