Re: [Xen-devel] blktap2 and CONFIG_XEN_BLKBACK_PAGEMAP

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 15.07.10 at 21:02, Kaushik Kumar Ram <kaushik@xxxxxxxx> wrote:
> It seems like on enabling CONFIG_BLKBACK_PAGEMAP the grant mechanism is used 
> to map guest pages into user space too. This means the guest pages are mapped 
> twice using the grant mechanism, first into dom0 kernel space (in 
> blkback/blback.c) and then into tapdisk process's address space (in 
> blktap2/device.c).  This is the new implementation of blkback.
> 
> On disabling CONFIG_BLKBACK_PAGEMAP, the code falls back on the old 
> implementation. Here, the guest pages are mapped into user space by directly 
> manipulating the page tables without going through the grant mechanism. 
> (Things seem slightly different when XENFEAT_auto_translated_physmap is set 
> but I will ignore that for now). First, does the old way still work? The 
> problem seems to arise when the page table entry is set in 
> blktap_umap_uaddr_fn() (in blktap2/device.c) which leads to a page fault and 
> Xen does not seem to like this page fault to handle it correctly and this 
> results in a panic. Why is the page table entry set directly without using a 
> hypercall here? 
> 
> Any further explanation will be much appreciated.

How could you have disabled XEN_BLKBACK_PAGEMAP in the first
place? It's a prompt-less option after all (for the very reason that
it's not optional).

Jan 


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