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

From: Ian Molton
Subject: Re: [Qemu-devel] Address translation - virt->phys->ram
Date: Mon, 22 Feb 2010 17:47:17 +0000
User-agent: Mozilla-Thunderbird (X11/20091109)

Anthony Liguori wrote:
> 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?

Because the buffers will all have to be copied. So far, switching from
abusing an instruction to interrupt qemu to using virtio has incurred a
roughly 5x slowdown. I'd guess much of this is down to the fact we have
to switch to kernel-mode on the guest and back again for every single GL

If I can establish some kind of stable guest_virt->phys->host_virt
mapping, many of the problems will just 'go away'. a way to interrupt
qemu from user-mode on the guest without involving the guest kernel
would be quite awesome also (theres really nothing we want the kernel to
actually /do/ here, it just adds overhead).


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