Re: [PATCH for-7.1] vfio/common: remove spurious tpm-crb-cmd misalignment warning

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 17 Mar 2022 15:34:53 +0100
Eric Auger <eric.auger@redhat.com> wrote:

> Hi Alex,
> 
> On 3/17/22 3:23 PM, Alex Williamson wrote:
> > On Thu, 17 Mar 2022 14:57:30 +0100
> > Eric Auger <eric.auger@redhat.com> wrote:
> >  
> >> Hi Alex,
> >>
> >> On 3/17/22 12:08 AM, Alex Williamson wrote:  
> >>> On Wed, 16 Mar 2022 21:29:51 +0100
> >>> Eric Auger <eric.auger@redhat.com> wrote:
> >>>    
> >>>> The CRB command buffer currently is a RAM MemoryRegion and given
> >>>> its base address alignment, it causes an error report on
> >>>> vfio_listener_region_add(). This region could have been a RAM device
> >>>> region, easing the detection of such safe situation but this option
> >>>> was not well received. So let's add a helper function that uses the
> >>>> memory region name to recognize the region and detect the situation
> >>>> is safe wrt assignment. Other regions can be listed here if such kind
> >>>> of problem occurs again.
> >>>>
> >>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>>> ---
> >>>>  hw/vfio/common.c     | 26 +++++++++++++++++++++++++-
> >>>>  hw/vfio/trace-events |  1 +
> >>>>  2 files changed, 26 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> >>>> index 080046e3f51..b58a38f5c57 100644
> >>>> --- a/hw/vfio/common.c
> >>>> +++ b/hw/vfio/common.c
> >>>> @@ -861,6 +861,22 @@ static void vfio_unregister_ram_discard_listener(VFIOContainer *container,
> >>>>      g_free(vrdl);
> >>>>  }
> >>>>  
> >>>> +static bool vfio_known_safe_misalignment(MemoryRegionSection *section)
> >>>> +{
> >>>> +    MemoryRegion *mr = section->mr;
> >>>> +
> >>>> +    if (strcmp(memory_region_name(mr), "tpm-crb-cmd") != 0) {
> >>>> +        return false;
> >>>> +    }    
> >>> Hi Eric,
> >>>
> >>> I was thinking more along the lines that we could use
> >>> memory_region_owner() to get the owning Object, then on
> >>> that we could maybe use INTERFACE_CHECK to look for TYPE_MEMORY_DEVICE,
> >>> then consider anything else optional.  (a) could something like that
> >>> work and (b) do all required mappings currently expose that interface?
> >>> Thanks,    
> >> If I understand correctly you just want to error_report() misalignement
> >> of MR sections belonging to
> >>
> >> TYPE_MEMORY_DEVICE devices and silence the rest? Is that a correct
> >> understanding? I thought you wanted to be much more protective and
> >> ignore misalignments on a case by case basis hence the white listing
> >> of this single tpm-crb-cmd region.  
> > Ah right, so I'm just slipping back into what we currently do, fail for
> > memory and warn on devices, which would be a generally reasonable long
> > term plan except people file bugs about those warnings.  Crud.
> >
> > I guess I don't have a better idea than creating essentially an
> > exception list like this.  Do you think it's better to do the strcmp
> > for the specific memory region or would it maybe be sufficient to test
> > the owner object is TYPE_TPM_CRB?  Thanks,  
> I asked myself the question and eventually chose to be more conservative
> with the granularity of the MR. Sometimes objects own several MRs and I
> wondered if some misalignments could be considered as safe while others
> unsafe, within the same object.  Nevertheless I don't have a strong
> opinion and will respin according to your preferencee.

Hi Eric,

As we discussed offline, I think the benefits of being able to test the
type, ie. TYPE_TPM_CRB, might outweigh the flexibility of having per mr
granularity.  The strcmp seems like a maintenance red flag since that's
subject to change, though maybe migration support forces it to be more
static than it would otherwise appear.  In any case, it's probably not
worth warning about any DMA mapping failures for mr's backed by a tpm
device.  Thanks,

Alex

> >>>> +
> >>>> +    /* this is a known safe misaligned region, just trace for
> >>>> debug purpose */
> >>>> +    trace_vfio_known_safe_misalignment(memory_region_name(mr),
> >>>> +
> >>>> section->offset_within_address_space,
> >>>> +
> >>>> section->offset_within_region,
> >>>> +                                       qemu_real_host_page_size);
> >>>> +    return true;
> >>>> +}
> >>>> +
> >>>>  static void vfio_listener_region_add(MemoryListener *listener,
> >>>>                                       MemoryRegionSection *section)
> >>>>  {
> >>>> @@ -884,7 +900,15 @@ static void
> >>>> vfio_listener_region_add(MemoryListener *listener, if
> >>>> (unlikely((section->offset_within_address_space &
> >>>> ~qemu_real_host_page_mask) != (section->offset_within_region &
> >>>> ~qemu_real_host_page_mask))) {
> >>>> -        error_report("%s received unaligned region", __func__);
> >>>> +        if (!vfio_known_safe_misalignment(section)) {
> >>>> +            error_report("%s received unaligned region %s
> >>>> iova=0x%"PRIx64
> >>>> +                         " offset_within_region=0x%"PRIx64
> >>>> +                         " qemu_real_host_page_mask=0x%"PRIxPTR,
> >>>> +                         __func__,
> >>>> memory_region_name(section->mr),
> >>>> +                         section->offset_within_address_space,
> >>>> +                         section->offset_within_region,
> >>>> +                         qemu_real_host_page_mask);
> >>>> +        }
> >>>>          return;
> >>>>      }
> >>>>  
> >>>> diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
> >>>> index 0ef1b5f4a65..6f38a2e6991 100644
> >>>> --- a/hw/vfio/trace-events
> >>>> +++ b/hw/vfio/trace-events
> >>>> @@ -100,6 +100,7 @@ vfio_listener_region_add_skip(uint64_t start,
> >>>> uint64_t end) "SKIPPING region_add vfio_spapr_group_attach(int
> >>>> groupfd, int tablefd) "Attached groupfd %d to liobn fd %d"
> >>>> vfio_listener_region_add_iommu(uint64_t start, uint64_t end)
> >>>> "region_add [iommu] 0x%"PRIx64" - 0x%"PRIx64
> >>>> vfio_listener_region_add_ram(uint64_t iova_start, uint64_t
> >>>> iova_end, void *vaddr) "region_add [ram] 0x%"PRIx64" - 0x%"PRIx64"
> >>>> [%p]" +vfio_known_safe_misalignment(const char *name, uint64_t
> >>>> iova, uint64_t offset_within_region, uint64_t page_size) "Region
> >>>> \"%s\" iova=0x%"PRIx64" offset_within_region=0x%"PRIx64"
> >>>> qemu_real_host_page_mask=0x%"PRIxPTR ": cannot be mapped for DMA"
> >>>> vfio_listener_region_add_no_dma_map(const char *name, uint64_t
> >>>> iova, uint64_t size, uint64_t page_size) "Region \"%s\"
> >>>> 0x%"PRIx64" size=0x%"PRIx64" is not aligned to 0x%"PRIx64" and
> >>>> cannot be mapped for DMA" vfio_listener_region_del_skip(uint64_t
> >>>> start, uint64_t end) "SKIPPING region_del 0x%"PRIx64" - 0x%"PRIx64
> >>>> vfio_listener_region_del(uint64_t start, uint64_t end) "region_del
> >>>> 0x%"PRIx64" - 0x%"PRIx64    
>