Re: R/W HG memory mappings with kvm?

[Avi Kivity <avi@redhat.com>]

On 07/06/2009 01:41 AM, Stephen Donnelly wrote:
> I am looking at how to do memory mapped IO between host and guests
> under kvm. I expect to use the PCI emulation layer to present a PCI
> device to the guest.
>
> I see virtio_pci uses cpu_physical_memory_map() which provides either
> read or write mappings and notes "Use only for reads OR writes - not
> for read-modify-write operations."
>    

Right, these are for unidirectional transient DMA.

> Is there an alternative method that allows large (Several MB)
> persistent hg memory mappings that are r/w? I would only be using this
> under kvm, not kqemu or plain qemu.
>    

All of guest memory is permanently mapped in the host.  You can use 
accessors like cpu_physical_memory_rw() or cpu_physical_memory_map() to 
access it.  What exactly do you need that is not provided by these 
accessors?

> Also it appears that PCI IO memory (cpu_register_io_memory) is
> provided via access functions, like the pci config space?

It can also use ordinary RAM (for example, vga maps its framebuffer as a 
PCI BAR).

> Does this
> cause a page fault/vm_exit on each read or write, or is it more
> efficient than that?
>    

It depends on how you configure it.  Look at the vga code (hw/vga.c, 
hw/cirrus_vga.c).  Also Cam (copied) wrote a PCI card that provides 
shared memory across guests, you may want to look at that.

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