Hi Bharat,
On 4/2/20 11:01 AM, Bharat Bhushan wrote:
> Hi Eric/Alex,
>
>> -----Original Message-----
>> From: Alex Williamson <alex.william...@redhat.com>
>> Sent: Thursday, March 26, 2020 11:23 PM
>> To: Auger Eric <eric.au...@redhat.com>
>> Cc: Bharat Bhushan <bbhush...@marvell.com>; peter.mayd...@linaro.org;
>> pet...@redhat.com; eric.auger....@gmail.com; kevin.t...@intel.com;
>> m...@redhat.com; Tomasz Nowicki [C] <tnowi...@marvell.com>;
>> drjo...@redhat.com; linuc.dec...@gmail.com; qemu-devel@nongnu.org; qemu-
>> a...@nongnu.org; bharatb.li...@gmail.com; jean-phili...@linaro.org;
>> yang.zh...@intel.com; David Gibson <da...@gibson.dropbear.id.au>
>> Subject: [EXT] Re: [PATCH v9 1/9] hw/vfio/common: Remove error print on mmio
>> region translation by viommu
>>
>> External Email
>>
>> ----------------------------------------------------------------------
>> On Thu, 26 Mar 2020 18:35:48 +0100
>> Auger Eric <eric.au...@redhat.com> wrote:
>>
>>> Hi Alex,
>>>
>>> On 3/24/20 12:08 AM, Alex Williamson wrote:
>>>> [Cc +dwg who originated this warning]
>>>>
>>>> On Mon, 23 Mar 2020 14:16:09 +0530
>>>> Bharat Bhushan <bbhush...@marvell.com> wrote:
>>>>
>>>>> On ARM, the MSI doorbell is translated by the virtual IOMMU.
>>>>> As such address_space_translate() returns the MSI controller MMIO
>>>>> region and we get an "iommu map to non memory area"
>>>>> message. Let's remove this latter.
>>>>>
>>>>> Signed-off-by: Eric Auger <eric.au...@redhat.com>
>>>>> Signed-off-by: Bharat Bhushan <bbhush...@marvell.com>
>>>>> ---
>>>>> hw/vfio/common.c | 2 --
>>>>> 1 file changed, 2 deletions(-)
>>>>>
>>>>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c index
>>>>> 5ca11488d6..c586edf47a 100644
>>>>> --- a/hw/vfio/common.c
>>>>> +++ b/hw/vfio/common.c
>>>>> @@ -426,8 +426,6 @@ static bool vfio_get_vaddr(IOMMUTLBEntry *iotlb,
>> void **vaddr,
>>>>> &xlat, &len, writable,
>>>>> MEMTXATTRS_UNSPECIFIED);
>>>>> if (!memory_region_is_ram(mr)) {
>>>>> - error_report("iommu map to non memory area %"HWADDR_PRIx"",
>>>>> - xlat);
>>>>> return false;
>>>>> }
>>>>>
>>>>
>>>> I'm a bit confused here, I think we need more justification beyond
>>>> "we hit this warning and we don't want to because it's ok in this
>>>> one special case, therefore remove it". I assume the special case
>>>> is that the device MSI address is managed via the SET_IRQS ioctl and
>>>> therefore we won't actually get DMAs to this range.
>>> Yes exactly. The guest creates a mapping between one giova and this
>>> gpa (corresponding to the MSI controller doorbell) because MSIs are
>>> mapped on ARM. But practically the physical device is programmed with
>>> an host chosen iova that maps onto the physical MSI controller's
>>> doorbell. so the device never performs DMA accesses to this range.
>>>
>>> But I imagine the case that
>>>> was in mind when adding this warning was general peer-to-peer
>>>> between and assigned and emulated device.
>>> yes makes sense.
>>>
>>> Maybe there's an argument to be made
>>>> that such a p2p mapping might also be used in a non-vIOMMU case. We
>>>> skip creating those mappings and drivers continue to work, maybe
>>>> because nobody attempts to do p2p DMA with the types of devices we
>>>> emulate, maybe because p2p DMA is not absolutely reliable on bare
>>>> metal and drivers test it before using it.
>>> MSI doorbells are mapped using the IOMMU_MMIO flag (dma-iommu.c
>>> iommu_dma_get_msi_page).
>>> One idea could be to pass that flag through the IOMMU Notifier
>>> mechanism into the iotlb->perm. Eventually when we get this in
>>> vfio_get_vaddr() we would not print the warning. Could that make sense?
>>
>> Yeah, if we can identify a valid case that doesn't need a warning, that's
>> fine by me.
>> Thanks,
>
> Let me know if I understood the proposal correctly:
>
> virtio-iommu driver in guest will make map (VIRTIO_IOMMU_T_MAP) with
> VIRTIO_IOMMU_MAP_F_MMIO flag for MSI mapping.
> In qemu, virtio-iommu device will set a new defined flag (say IOMMU_MMIO) in
> iotlb->perm in memory_region_notify_iommu(). vfio_get_vaddr() will check same
> flag and will not print the warning.>
> Is above correct?
Yes that's what I had in mind.
Thanks
Eric
>
> Thanks
> -Bharat
>
>>
>> Alex
>
>
