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Re: [Xen-devel] sharing memory between qemu and domu without grant table

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] sharing memory between qemu and domu without grant tables or vmexits...
From: Keir Fraser <keir@xxxxxxx>
Date: Sun, 17 Oct 2010 08:36:56 +0100
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Thread-topic: [Xen-devel] sharing memory between qemu and domu without grant tables or vmexits...
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On 17/10/2010 06:27, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

> Is it possible to share a page of memory between qemu in dom0 and domu
> without using grant tables and without domu access to that page causing
> a vmexit?

Pick a reserved page in guest's physical memory map. Communicate that
address to both domU and qemu somehow (e.g., one might pick it and
communicate it to the other; or it may be a statically assigned well-known
address; or whatever). Qemu can map that page, as obviously can the guest.

To get that reserved page, you could: (1) pick an unused frame slot in the
hole below 4GB (e.g., between LAPIC and IOAPIC mappings) allocate a page
there from qemu, or hvmloader, or domU, or...; or (2) you could have hvm
domain builder, or hvmloader, reserve a page from the guest's regular RAM
space, and update guest e820 appropiately to indicate page is reserved.
Communicate that address as necessary, and away you go.

 -- Keir

> To set up a custom kernel debug dll under windows I need to load very
> early, much earlier than my PV drivers. To have any hope of usefully
> increasing the speed over serial port debugging I need to be able to
> transfer data faster than 1-byte-per-vmexit...
> Thanks
> James
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