Multiple virtual mappings considered harmful on ARM

Marcel Moolenaar xcllnt at
Fri Dec 19 18:30:40 UTC 2008

On Dec 19, 2008, at 6:07 AM, Grzegorz Bernacki wrote:

> 2. Root cause.
> The root cause of the problem is additional virtual mapping of read/ 
> write
> buffers at cluster read/write (sys/kern/vfs_cluster.c,  
> cluster_rbuild(),
> cluster_wbuild(). Buffers for sequential read/write operation are  
> concatenated
> and sent to device as one big buffer. Concatenation of buffers uses
> pmap_qenter(), which puts *additional* mapping in the KVA for  
> physical area
> already mapped. For each buffer we extract pages it contains and  
> then all the
> pages from all the buffers are mapped into new virtual address of  
> new buffer.
> So we end up with at least two virtual addresses for each page.

Could this also affect I-cache coherency by virtue of not
flushing the D-cache properly before synchronizing the
I-cache, as you mention reading?

Marcel Moolenaar
xcllnt at

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