Re: R/W HG memory mappings with kvm?

[Cam Macdonell <cam@cs.ualberta.ca>]

Avi Kivity wrote:
> On 07/08/2009 01:23 AM, Stephen Donnelly wrote:
>>
>>>> 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).
>>>      
>>
>> So host memory is exported as a PCI_BAR to the guest via
>> cpu_register_physical_memory(). It looks like the code has to
>> explicitly manage marking pages dirty and synchronising at appropriate
>> times. Is the coherency problem bidirectional, e.g. writes from either
>> host or guest to the shared memory need to mark pages dirty, and
>> ensure sync is called before the other side reads those areas?
>>    
> 
> Shared memory is fully coherent.  You can use the ordinary x86 bus lock 
> operations for concurrent read-modify-write access, and the memory 
> barrier instructions to prevent reordering.  Just like ordinary shared 
> memory.
> 
>>>> 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.
>>>      
>>
>> I will look into the vga code and see if I get inspired. The 'copied'
>> driver sounds interesting, the code is not in kvm git?
>>    
> 
> (copied) means Cam was copied (cc'ed) on the email, not the name of the 
> driver.  It hasn't been merged but copies (of the driver, not Cam) are 
> floating around on the Internet.

Hi Stephen,

Here is the latest patch that supports interrupts.  I am currently 
working on a broadcast mechanism that should be ready fairly soon.

http://patchwork.kernel.org/patch/22368/

I have some test scripts that can demonstrate how to use the memory 
between guest/host and guest/guest.  Let me know if you would like me to 
send them to you.

Cheers,
Cam

> 
> The relevant parts of cirrus_vga.c are:
> 
> static void cirrus_pci_lfb_map(PCIDevice *d, int region_num,
>                                uint32_t addr, uint32_t size, int type)
> {
> 
> ...
> 
>     /* XXX: add byte swapping apertures */
>     cpu_register_physical_memory(addr, s->vga.vram_size,
>                                  s->cirrus_linear_io_addr);
> 
> This function is called whenever the guest updates the BAR.
>