Re: [Xen-devel] Mapping another domain's user memory

[Michael Abd-El-Malek <mabdelmalek@xxxxxxx>]

On Jan 31, 2008, at 11:16 AM, Derek Murray wrote:

> On Jan 31, 2008 2:57 PM, Michael Abd-El-Malek <mabdelmalek@xxxxxxx>  
> wrote:
> > I tried to do that by first getting a grant for the user-space memory
> > in the source domain as follows:
> >
> >        page_start = user_buf;
> >        buffer_mfn = virt_to_mfn(page_start);
> >        gnttab_grant_foreign_access_ref(grant_ref, info->dev- 
> > >otherend_id,
> > buffer_mfn, 0);
> >
> > But the virt_to_mfn call asserted, in include/asm-x86_64/mach-xen/ 
> > asm/
> > maddr.h:pfn_to_mfn.
> > The assertion that failed was: BUG_ON(end_pfn && pfn >= end_pfn);
> You have another problem here, because virt_to_mfn (and virt_to_page
> etc.) operate on kernel virtual addresses, where converting to a
> physical address effectively just involves subtracting an offset. User
> virtual addresses are handled differently, and you'll have to walk the
> page table to find the PTE and from that obtain an MFN.
Thanks for your help Derek.  Fortunately I didn't have to muck around  
with the page tables.  get_user_pages already does this.  So I called  
that function to get the relevant pages for the user pointer, kmap'ed  
those pages, got the virtual address using virt_to_mfn, and Xen's  
grants worked.
Cheers,
Mike

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