Date: Mon, 24 Feb 2003 09:21:58 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: Byunghyun Oh <octphial@postech.ac.kr> Cc: freebsd-hackers@freebsd.org Subject: Re: Replacement for get_user_pages() of Linux Message-ID: <3E5A54B6.CC1639F1@mindspring.com> References: <20030223163746.A19421@shell.postech.ac.kr> <20030224100346.GA6374@HAL9000.homeunix.com> <3E59FA6C.CF8CB852@mindspring.com> <20030224143911.GA645@localhost.postech.ac.kr>
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Byunghyun Oh wrote:
> ps. Is DMA means just 'Direct Memory Access', and shall I understand it
> literally?
Yes, DMA stands for "Direct Memory Access".
The purpose of DMA is to allow devices other than the main CPU
to directly access regions of physical memory by using a DMA
line (DRQ) to arbitrate access to the physical memory bus, and
then perform reads (DMA out) or writes (DMA in) to physical
RAM belonging to system memory, rather than to the device.
The reason to do permit this is so that the main CPU can keep
processing whatever is in it's I and D (Instruction and Data)
caches, without introducing a stall while the main CPU does its
work. In this way, copy processing can be offloaded from the
main CPU to chips on devices ("DMA engines"), instead.
-
In the context of Plex86, the reason for having this would be
to permit physical devices to be assigned to the virtual machine,
and not interact with the host system (I think). This idea ia
somewhat flawed, and I don't think you can avoid the host system
interaction. VMWare handles this by creating virtual hardware
that then bridges to the real hardware (e.g. for serial ports and
network adapters).
In any case, what this would imply is that the kernel running
under the Plex86 that has had the resource assigned to it would
need *its* pages wired down, so that *its* driver(s) for the real
hardware that has been assigned to it can pass "kernel" addresses
to the DMA engines on the hardware, and have the hardware able to
DMA into the "kernel" memory for the host OS.
It seems to me that direct assignment of hardware resources is a
bad idea, in this context, and that additional modifications to
the hosted OS would be required in order to permit these addresses
to be exported to real hardware, for DMA purposes. Specifically,
you would need to translate from the virtual address to the
physical address, given that the physical address is virtualized
by the hardware, and this would not be automatic.
A useful resource for someone porting Plex86 to FreeBSD is probably
the NetBSD port of the code, back in December of last year (though
the Plex86 web site claims that the code is significantly changed,
as of February of this year, to mostly run in user space). See:
http://slashdot.org/article.pl?sid=00/12/14/1041230
Also, FWIW, it looks like it already runs on FreeBSD?!?:
<http://savannah.nongnu.org/cgi-bin/viewcvs/plex86/plex86-release/kernel/host-freebsd.c>;
<http://savannah.nongnu.org/cgi-bin/viewcvs/*checkout*/plex86/plex86-release/kernel/host-freebsd.c?rev=1.1.1.1&content-type=text/plain>;
-- Terry
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