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

Anthony Liguori
2008-12-12T19:39:33Z

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 no longer valid.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