Re: [Qemu-devel] [RFC] Memory API

[Gleb Natapov]

On Thu, May 19, 2011 at 02:44:29PM +0300, Avi Kivity wrote:
> On 05/19/2011 12:14 PM, Gleb Natapov wrote:
> >On Thu, May 19, 2011 at 12:10:38PM +0300, Avi Kivity wrote:
> >>  On 05/19/2011 12:08 PM, Gleb Natapov wrote:
> >>  >On Wed, May 18, 2011 at 06:42:14PM +0300, Avi Kivity wrote:
> >>  >>   On 05/18/2011 06:36 PM, Jan Kiszka wrote:
> >>  >>   >>
> >>  >>   >>    We need to head for the more hardware-like approach.  What 
> >> happens when
> >>  >>   >>    you program overlapping BARs?  I imagine the result is
> >>  >>   >>    implementation-defined, but ends up with one region decoded in
> >>  >>   >>    preference to the other.  There is simply no way to reject an
> >>  >>   >>    overlapping mapping.
> >>  >>   >
> >>  >>   >But there is also now simple way to allow them. At least not without
> >>  >>   >exposing control about their ordering AND allowing to hook up 
> >> managing
> >>  >>   >code (e.g. of the PCI bridge or the chipset) that controls 
> >> registrations.
> >>  >>
> >>  >>   What about memory_region_add_subregion(..., int priority) as I
> >>  >>   suggested in another message?
> >>  >Haven't saw another message yet, but how caller knows about priority?
> >>
> >>  The caller is emulating some hub or router and should decide on
> >>  priority like real hardware.
> >>
> >>  For example, piix gives higher priority to the vga window over RAM.
> >>
> >Hmm, but if a caller of the memory_region_add_subregion() function is a
> >device itself how does it know about chipset priorities. All it wants to
> >tell to the system is that it is ready to handle mmio access in this phys
> >range, but chipset may decide to forward those accesses elsewhere.
> 
> In this case the device would call a chipset function, passing the
> memory region as a parameter, and the chipset would call
> m_r_add_subregion().
But then chipset can resolve all overlapping by itself and register only
regions that are actually accessible by a guest software. Also there are
devices that on some architectures are accessed through a chipset and on
other they resides directly on a system bus. If they will need to call
different memory registration api depending on how they are instantiated
the code can become messy.

>                       Alternatively, the chipset can instantiate the
> device (if it is an embedded one) and call m_r_add_subregion()
> itself.
> 

--
                        Gleb.