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[Qemu-discuss] Debugging fw_cfg DMA code

From: Adam Parker
Subject: [Qemu-discuss] Debugging fw_cfg DMA code
Date: Thu, 2 May 2019 13:50:13 -0500

Hello all,

I am attempting to write a driver for a guest OS to grab and set
values in fw_cfg.  I have used official documentation, osdev.org wiki
and the Linux kernel as inspiration.  I have checked the signature,
both normal and DMA signatures are present.  I have used the file
directory facilities successfully and recovered a key for the file I
want.  I then attempt a read against that file via DMA.  I realize
these operations are supposed to be synchronous for now but I "wait"
for the control bits to be unset before continuing (practically the
same code as Linux kernel).  Unfortunately this never happens and
there is no indication as to why or even that QEMU recognizes I'm
trying to do something.

I found the trace command line arguments and I turned on all related
to fw_cfg but only see activity related to the normal reads present in
signature/file directory.  Are there any that can be used to debug the
DMA portion of fw_cfg?

Are there any common pitfalls?  My code looks exactly like everyone
else's and it just doesn't work so I'm going a bit crazy.  I am
primarily testing against 3.0.93 (v3.1.0-rc3-dirty) and occasionally
running 4.0.0-rc4 as a sanity check.  Here is a link to a thread
discussing this effort plus code as of yesterday:

Adam Parker

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