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Re: [PATCH -tip 0/6 V4] tracing: kprobe-based event tracer


Pekka Paalanen wrote:
Not just emulation but address diversion, i.e. modifying the operation
(not the text) before executing it. Mmiotrace could do something like
this:
1. a blob calls ioremap
2. mmiotrace maps the MMIO area privately
3. the blob receives a dummy map from ioremap, that will generate
page fault
4. the blob accesses the dummy map and raises a page fault
5. pf handler detects the dummy map
6. mmiotrace pf handler emulates the instruction and replaces the
dummy address with the real MMIO address.
7. mmiotrace records the operation and the datum
8. go to step 4, or whatever

This means mmiotrace would not have to fiddle with the page
tables and page presence bits like it does now. As said, this
would make mmiotrace SMP-proof, and also eliminate the die notifier
(used for the instruction single stepping trap).

IMO a big step from a hack to a tool. Getting rid of the custom
instruction parser in mmiotrace would be a good step in itself.

Avi Kivity noted, that the KVM emulator does almost everything. Does
it allow also address diversion?

Operand access is by means of a callback, so yes. In kvm's use, it's used to access guest memory, so it modified the addresses before reading or writing.


--
error compiling committee.c: too many arguments to function


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