Re: [PATCH V2] intel_iommu: refine iotlb hash calculation

[Peter Xu]

On Wed, Apr 12, 2023 at 03:35:10PM +0800, Jason Wang wrote:
> Commit 1b2b12376c8 ("intel-iommu: PASID support") takes PASID into
> account when calculating iotlb hash like:
> 
> static guint vtd_iotlb_hash(gconstpointer v)
> {
>     const struct vtd_iotlb_key *key = v;
> 
>     return key->gfn | ((key->sid) << VTD_IOTLB_SID_SHIFT) |
>            (key->level) << VTD_IOTLB_LVL_SHIFT |
>            (key->pasid) << VTD_IOTLB_PASID_SHIFT;
> }
> 
> This turns out to be problematic since:
> 
> - the shift will lose bits if not converting to uint64_t
> - level should be off by one in order to fit into 2 bits
> - VTD_IOTLB_PASID_SHIFT is 30 but PASID is 20 bits which will waste
>   some bits
> - the hash result is uint64_t so we will lose bits when converting to
>   guint
> 
> So this patch fixes them by
> 
> - converting the keys into uint64_t before doing the shift
> - off level by one to make it fit into two bits
> - change the sid, lvl and pasid shift to 26, 42 and 44 in order to
>   take the full width of uint64_t
> - perform an XOR to the top 32bit with the bottom 32bit for the final
>   result to fit guint
> 
> Fixes: Coverity CID 1508100
> Fixes: 1b2b12376c8 ("intel-iommu: PASID support")
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu