Re: [Qemu-devel] Address translation - virt->phys->ram

[Anthony Liguori]

On 02/22/2010 07:59 AM, Ian Molton wrote:
> Hi folks,
>
> I've been updating some old patches which make use of a function to
> translate guest virtual addresses into pointers into the guest RAM.
>
> As I understand it qemu has guest virtual and physical addresses, the
> latter of which map somehow to host ram addresses.
>
> The function which the code had been using appears not to work under
> kvm, which leads me to think that qemu doesnt emulate the MMU (or at
> least not in the same manner) when it is using kvm as opposed to pure
> emulation.
>
> If I turn off kvm, the patch works, albeit slowly. If I enable it, the
> code takes the path which looks for the magic value (below).
>
> Is there a 'proper' way to translate guest virtual addresses into host
> RAM addresses?
>    

cpu_physical_memory_map().

But this function has some subtle characteristics.  It may return a 
bounce buffer if you attempt to map MMIO memory.  There is a limited 
pool of bounce buffers available so it may return NULL in the event that 
it cannot allocate a bounce buffer.

It may also return a partial result if you're attempting to map a region 
that straddles multiple memory slots.

Regards,

Anthony Liguori