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Date: Fri, 12 Dec 2008 13:15:13 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: chrisw@redhat.com, avi@redhat.com, Gerd Hoffmann <kraxel@redhat.com>, kvm@vger.kernel.org, qemu-devel@nongnu.org

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no longer valid.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);

The whole dma.c thing should not exist.  If we're going to introduce a 
higher level API, it should be a PCI DMA API.

Something like virtio could use this API directly seeing as it doesn't 
really live on a PCI bus in real life.

Regards,

Anthony Liguori