Re: [Qemu-devel] Address translation - virt->phys->ram

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/22/2010 10:46 AM, Ian Molton wrote:
> Anthony Liguori wrote:
>
>    
>> cpu_physical_memory_map().
>>
>> But this function has some subtle characteristics.  It may return a
>> bounce buffer if you attempt to map MMIO memory.  There is a limited
>> pool of bounce buffers available so it may return NULL in the event that
>> it cannot allocate a bounce buffer.
>>
>> It may also return a partial result if you're attempting to map a region
>> that straddles multiple memory slots.
>>      
> Thanks. I had found this, but was unsure as to wether it was quite what
> I wanted. (also is it possible to tell when it has (eg.) allocated a
> bounce buffer?)
>
> Basically, I need to get buffer(s) from guest userspace into the hosts
> address space. The buffers are virtually contiguous but likely
> physically discontiguous. They are allocated with malloc() and theres
> nothing I can do about that.
>
> The obvious but slow solution would be to copy all the buffers into nice
> virtio-based scatter/gather buffers and feed them to the host that way,
> however its not fast enough.
>    

Why is this slow?

Regards,

Anthony Liguori

> Right now I have a little driver I have written that allows a buffer to
> be mmap()ed by the guest userspace, and this is pushed to the host via
> virtio s/g io when the guest calls fsync(). This buffer contains the
> data that must be passed to the host, however this data may often
> contain pointers to (that is, userspace virtual addresses of) buffers of
> unknown sizes which the host also needs to access. These buffers are
> what I need to read from the guests RAM.
>    



> The buffers will likely remain active across multiple different calls to
> the host so their pages will need to be available. As the calls always
> happen when that specific process is active, I'd guess the worst we need
> to do is generate a page fault to unswap the page(s). Can that be caused
> by qemu (under kvm)?
>
> It seems that cpu_physical_memory_map() deals with physically contiguous
> areas of guest address space. I need to get a host-side mapping of a
> *virtually* contiguous (possibly physically discontiguous) set of guest
> pages. If this can be done, it'd mean direct transfer of data from guest
> application to host shared library, which would be a major win.
>
> -Ian
>