[Xen-devel] dom0 boot failure: dma_reserve in reserve_bootmem_generic()

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

Hi,

I've been debugging an interesting dom0 boot failure which happens
only on certain machines when dom0_mem=830M. A scsi driver fails to 
allocate 512 bytes in GFP_DMA. However, the system boots with 500M or
930M.  The root cause:

reserve_bootmem_generic():
....
        if (phys+len <= MAX_DMA_PFN*PAGE_SIZE)
                dma_reserve += len / PAGE_SIZE;      <---


In case of 830M, phys+len is just enough to set dma_reserve where
the dma memory zone is then 'holed' out. With less dom0_mem, it
leaves few pages for GFP_DMA. With more, phys+len is larger than 
MAX_DMA_PFN*PAGE_SIZE and it skips setting dma_reserve so DMA cache is
good again.

So, anyone know the point of setting dma_reserve? Obviously, things 
are implied OK without it, so would it be safe to just remove the
if stmt completely?

thanks a lot,
Mukesh


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