Re: [Qemu-devel] [RFC v2 00/20] Memory API

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 27, 2011 at 06:54:05PM +0300, Avi Kivity wrote:
> On 06/27/2011 06:52 PM, Michael S. Tsirkin wrote:
> >On Mon, Jun 27, 2011 at 06:13:03PM +0300, Avi Kivity wrote:
> >>  On 06/27/2011 04:21 PM, Avi Kivity wrote:
> >>  >As expected, this is taking longer than expected, so I'm releasing something
> >>  >less complete than I'd have liked.  Not even all of the PC machine is
> >>  >converted, but the difficult parts are (cirrus).  It appears to work well.
> >>  >
> >>  >The major change compared to v1 is the introduction of
> >>  >memory_region_init_alias(), which defines a memory region in terms of another.
> >>  >With the current API, the ability to alias is provided by address arithmetic
> >>  >on ram_addr_t:
> >>  >
> >>  >    ram_addr = qemu_ram_alloc(...);
> >>  >    cpu_register_physical_memory(..., ram_addr, size, ...);
> >>  >    /* alias: */
> >>  >    cpu_register_physical_memory(..., ram_addr + offset, another_size, ...);
> >>  >
> >>  >With the new API, you have to create an alias:
> >>  >
> >>  >    memory_region_init_ram(&mem, ...);
> >>  >    memory_region_register_subregion(...,&mem);
> >>  >    /* alias: */
> >>  >    memory_region_init_alias(&alias, ...,&mem, offset, another_size);
> >>  >    memory_region_register_subregion(...,&alias);
> >>  >
> >>  >The patchset is somewhat churny.  One of the reasons is that we move from a
> >>  >handle/pointer scheme in ram_addr_t to an object constructor/destructor scheme.
> >>  >Another is that region size becomes a property of a region instead of being
> >>  >maintained externally.  Also, container memory regions must be passed around,
> >>  >though we don't do that as well as we should.
> >>  >
> >>  >Todo:
> >>  >    - eliminate calls to get_system_memory() (where we ignore the bus hierarchy)
> >>  >    - add PCI APIs for the VGA window
> >>  >    - support the PIO address space using the memory API (allowing simplified
> >>  >      PCI BAR registration)
> >>  >    - convert 440FX
> >>  >    - convert everything else
> >>
> >>  Michael, I'm looking at the pci bridge code, and it basically does
> >>  the same thing - clip each BAR to the intersection of the decode
> >>  window of all bridges it hides behind.
> >>
> >>  How does a PCI bridge behave wrt VGA?  is it a separate control?  If
> >>  so I probably need to implement generalized clipping (i.e. decode
> >>  between 0xa0000-0xc0000 or between 0xc0000000-0xf0000000).
> >
> >Per spec, VGA is special:
> >- bridges have VGA enable bit
> >
> >The VGA Enable bit in the Bridge Control register (see Section 3.2.5.18)
> >is used to control response by the bridge to both the VGA frame buffer
> >addresses and to the VGA register addresses. When a VGA compatible
> >device is located downstream of a PCI-to-PCI bridge, the VGA Enable bit
> >must be set. When set, the bridge will positively decode and forward
> >memory accesses to VGA frame buffer addresses and I/O accesses to VGA
> >registers from the primary to secondary interface and block forwarding
> >of these same accesses from the secondary to primary interface (see
> >Section 4.5.1).
> >VGA memory addresses:
> >0A 0000h through 0B FFFFh
> >VGA I/O addresses (including ISA aliases address - AD[15::10] are not
> >decoded):
> >AD[9::0] = 3B0h through 3BBh and 3C0h through 3DFh
> >The bridge does not decode or forward VGA BIOS memory addresses when the
> >VGA Enable bit is set. ROM code provided by PCI compatible devices may
> >be mapped to any address in PCI memory address space via the Expansion
> >ROM Base Address register in the device's configuration header and must
> >be copied to system memory before execution.
> >
> 
> Okay, looks like generalized clipping is needed.  It's nice anyway
> and guarantees me a job for life as maintainer of memory.c.
> 
> 
> >- bridges might also enable subtractive decoding
> >   (required for isa behind the bridge)
> 
> What does that mean?

subtractive decoding is 
a method of address decoding in which a device accepts all
accesses not positively decoded by another agent.



> -- 
> error compiling committee.c: too many arguments to function