[Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO

[Anthony Liguori <anthony@codemonkey.ws>]

Anthony Liguori wrote:
> Andrea Arcangeli wrote:
>> From: Andrea Arcangeli <aarcange@redhat.com>
>>
>> Add can_dma and post_dma methods needed before/after direct IO to guest
>> physical memory.
>>   
>
> I think any API based on a can_dma abstraction is wrong.  The 
> write_post_dma thing is equally wrong.
>
> The concept of "dma" that you're introducing is not correct.
>
> The DMA API should have the following properties:
>
> 1) You attempt to map a physical address.  This effectively is a lock 
> or pin operation.
>  a) In the process of this, you get a virtual address that you can 
> manipulate.
> 2) You do your IO to the virtual address
> 3) You indicate how much of the memory you have dirtied
> 4) You unmap or unlock that memory region.  The virtual address is now 
> no longer valid.
>
> This could correspond to a:
>
> void *cpu_physical_memory_map(target_phys_addr_t addr, ram_addr_t 
> size, int is_write);
>
> void cpu_physical_memory_unmap(target_physical_addr_t addr, ram_addr_t 
> size, void *mapping, int is_dirty);

Let me clarify this a bit more.  The problem we're trying to address 
today is the encapsulating knowledge of phys_ram_base.  We want to 
minimize the amount of code that makes any assumptions about 
phys_ram_base.  Your current API still accesses phys_ram_base directly 
in the PCI DMA API.  The only real improvement compared to the current 
virtio code is that you properly handle MMIO.  This is not just about 
layout but this also includes the fact that in the future, guest memory 
could be discontiguous in QEMU (think memory hotplug).

Or, in the case of Xen, it may not be possible to have all of guest 
memory mapped at any given time.  The API that I propose above limits 
the knowledge of how to access guest memory to exec.c and makes it very 
easy to deal with discontiguous or even partially mapped guest memory in 
QEMU.  Any other APIs built around it (like a PCI DMA API) don't have to 
have any special knowledge about how guest memory is stored.

Regards,

Anthony Liguori