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

[Avi Kivity <avi@redhat.com>]

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