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Date: Fri, 24 Apr 2015 12:52:59 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] physical memory address in guest (from his point
	of view)
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To: Catalin Vasile <catalinvasile92@gmail.com>
Cc: qemu-devel@nongnu.org



On 24/04/2015 12:22, Catalin Vasile wrote:
> > > Does a physical memory address in a guest points to the same memory as qemu?
> > > For example: if I have a physical pointer 0x34 in the guest, does it
> > > point to the same location that qemu points with 0x34 address?
> >
> > No.  QEMU converts guest pointers (which are just uint64_t) to host
> > pointers using address_space_map/address_space_unmap.
>
> That being said. Does a message that passes from guest to a backend
> need to pass through qemu everytime?
> I'm trying to understand how virtio virtqueues work. Namely I'm trying
> to pass as data itself a pointer in which the backend will write,
> namely use copy_to_user() if the backend is a kernel module.

vhost provides the infrastructure to do the same thing as
address_space_map/unmap.

Passing pointers inside a data buffer would be very weird for virtio
devices.  The way you do it typically, is that you pass a one descriptor
corresponding to a request or response header, and one or more
descriptors for the data buffers.

Paolo