Re: [Qemu-devel] [PATCH v3 04/12] memory: fix address_space_get_iotlb_entry()

[David Gibson]

On Wed, May 10, 2017 at 04:01:47PM +0800, Peter Xu wrote:
> This function has an assumption that we will definitely call translate()
> once (or say, the addr will be located inside one IOMMU memory region),
> otherwise an empty IOTLB will be returned. Nevertheless, this is not
> what we want. When there is no IOMMU memory region, we should build up a
> static mapping for the caller, instead of an invalid IOTLB.
> 
> We won't trigger this path before VT-d passthrough mode. When
> passthrough mode for a vhost device is setup, VT-d is possible to
> disable the IOMMU region for that device. Without current patch, we'll
> get a vhost boot failure, and it'll be failed over to virtio userspace
> mode.

This doesn't look right to me.  You're assuming the target is
address_space_memory, which might not be the case - and you should be
able to check from the MR you do hit.  Furthermore it doesn't look
like you're accounting for the trivial translation if the section's
offset in the address space is different from its offset in the MR.

I think for the fallback path you're going to want something based on
address_space_translate() instead.

> CC: Paolo Bonzini <address@hidden>
> CC: Jason Wang <address@hidden>
> CC: Michael S. Tsirkin <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  exec.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/exec.c b/exec.c
> index 072de5d..5cfdacd 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -463,12 +463,13 @@ address_space_translate_internal(AddressSpaceDispatch 
> *d, hwaddr addr, hwaddr *x
>  }
>  
>  /* Called from RCU critical section */
> -IOMMUTLBEntry address_space_get_iotlb_entry(AddressSpace *as, hwaddr addr,
> +IOMMUTLBEntry address_space_get_iotlb_entry(AddressSpace *as, hwaddr iova,
>                                              bool is_write)
>  {
>      IOMMUTLBEntry iotlb = {0};
>      MemoryRegionSection *section;
>      MemoryRegion *mr;
> +    hwaddr addr = iova, psize;
>  
>      for (;;) {
>          AddressSpaceDispatch *d = atomic_rcu_read(&as->dispatch);
> @@ -478,6 +479,23 @@ IOMMUTLBEntry address_space_get_iotlb_entry(AddressSpace 
> *as, hwaddr addr,
>          mr = section->mr;
>  
>          if (!mr->iommu_ops) {
> +            /*
> +             * We didn't translate() but reached here. It possibly
> +             * means it's a static mapping. If so (it should be RAM),
> +             * we set the IOTLB up.
> +             */
> +            if (!iotlb.target_as && memory_region_is_ram(mr) &&
> +                !memory_region_is_rom(mr)) {
> +                psize = mr->ram_block->page_size;
> +                iova &= ~(psize - 1);
> +                iotlb = (IOMMUTLBEntry) {
> +                    .target_as = &address_space_memory,
> +                    .iova = iova,
> +                    .translated_addr = iova,
> +                    .addr_mask = psize - 1,
> +                    .perm = IOMMU_RW,
> +                };
> +            }
>              break;
>          }
>  

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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