Re: [Qemu-devel] [PATCH 1/5] Add target memory mapping API

[Avi Kivity <avi@redhat.com>]

Jamie Lokier wrote:
> Avi Kivity wrote:
>   
>> In fact, we could even say that the virtual hardware doesn't support 
>> dma-to-mmio at all and MCE the guest.  I'm sure no x86 guest would even 
>> notice.  Don't know about non-x86.
>>     
>
> Guest userspace does:
>
>    1. mmap() framebuffer device.
>    2. read() from file opened with O_DIRECT.
>
> Both are allowed by non-root processes on Linux.
>
> (I imagine this might be more common in some obscure DOS programs though).
>
> Think also variation with reading from a video capture device into
> video memory.  I've seen that done on x86, never seen it (yet)
> on non-x86 :-)
>
> However, that is known to break on some PCI bridges.
>
> I'm not sure if it's reasonable to abort emulation with an MCE in this
> case.
>
>   

Framebuffers are mapped as RAM, so we won't bounce this case. Try harder :)

>>> I think my question about partial DMA writes is very relevant.  If we
>>> don't care about that, nor about the corresponding notification for
>>> reads, then the API can be a lot simpler.
>>>       
>> I don't see a concrete reason to care about it.
>>     
>
> Writing zeros or junk after a partial DMA is quite different to real
> hardware behaviour.  Virtually all devices with a "DMA count"
> register are certain to have not written to a later address when DMA stops.
>
>   

The devices we're talking about here don't have a DMA count register. 
They are passed scatter-gather lists, and I don't think they make 
guarantees about the order in which they're accessed.

> QEMU tries to do a fairly good job at emulating devices with many of
> their quirks.  It would be odd if the high-performance API got in the
> way of high-quality device emulation, when that's wanted.
>
> Potential example: If a graphics card or video capture card, or USB
> webcam etc. (more likely!) is doing a large streaming DMA into a
> guests's userspace process when that process calls read() (in the
> guest OS), and the DMA is stopped for any reason, such as triggered by
> a guest OS SIGINT or simply the data having ended, the guest's
> userspace can reasonably assume data after the count returned by
> read() is untouched.
>   

This DMA will be into RAM, not mmio.

> Just as importantly, the guest OS in that example can assume that the
> later pages are not dirtied, therefore not swap them, or return them
> to its pre-zero pool or whatever.  This is a legitimate guest OS
> optimisation for streaming-DMA-with-unknown-length devices.  This can
> happen without a userspace process too.
>
> I'm guessing truncated DMAs using this API are always going to dirty
> only an initial part of the buffer, not arbitrary regions.  (In rare
> cases where this isn't true, don't use the API).
>
> So wouldn't it be trivial to pass "amount written" to the unmap
> function - to be used in the bounce buffer case?
>   

We don't have a reliable amount to pass.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.