[Qemu-devel] Re: Add target memory mapping API

[Mike Day]

On 21/01/09 17:09 +0000, Ian Jackson wrote:
> Anthony Liguori writes ("[Qemu-devel] Re: [PATCH 1/5] Add target memory 
> mapping API"):
> > I've gone though the thread again and I think this patch series is a 
> > pretty good start.  I think the API can be refined a bit more down the 
> > road to better support Xen but I think this is a good start.
> 
> I'm afraid I disagree.
> 
> > Does anyone have major blockers with this API?  If not, I'd like to 
> > commit these patches.  The consumers of it should be small provided that 
> > we make sure to write helpers for the various types of IO pipelines.
> 
> I have three criticisms, the first of which is in my view a major
> blocker:
> 
>  * The unmap call should take a transfer length, for the reasons
>    I have explained.
> 
>    This is absolutely critical for correctness if bounce buffers are
>    used.
> 
>    I think it is important that the API permits implementations where
>    the memory cannot be just mapped.  At the moment the APIs used by
>    qemu device emulations do not assume that they can access RAM
>    willy-nilly; they are all expected to go through
>    cpu_physical_memory_rw.  I think it is important to preserve this
>    for the future flexibility of the qemu codebase.

cpu_physical_memory_rw will still be available and used by most
devices even with this proposed API, right? Certain devices will be
able to now use the new API.
 
>    Note that Xen is not such an implementation, although in the Xen
>    case the additional performance of mapping is not so important so
>    we might not choose to implement the mapping right away.  Obviously
>    it would be much easier for us just to implement this mapping than
>    to argue at length on this list.  It's not hard for Xen.  So I'm
>    not trying to special-case Xen here.  You shouldn't read my
>    criticisms as some kind of Xen fanatic trying to defend some crazy
>    Xen behaviour.
> 
>    I'm trying to preserve an important and useful property of the
>    internal qemu API.  My suggestion will mean the device emulation
>    parts of qemu continue to be reasonably easily useable separately
>    from the rest of qemu, and possibly very separately from any
>    emulation of CPU or memory.  The benefit is difficult to evaluate
>    exactly but the cost is very small.
> 
> Of my criticisms, I think this is by far the most important.  It
> relates to the correctness of the API and in my view it is essential
> that this be properly addressed.
> 
> Avi will say again (as if repetition made things true) that `bounce
> buffers are never used' but of course they will be sometimes.  If they
> were never used his patch wouldn't need to have that code in them.
> It's true that they will be _rarely_ used but a lack of correctness in
> a rare case is not acceptable either.

I think a DMA into a device MMIO range is a special case.


Mike
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Mike Day
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