Re: [Question] Custom MMIO handler - is it possible?

[Mateusz Nowicki <nowicki@posteo.net>]

Thanks for a quick reply Bjorn!

Actually performance is not the biggest concern.
Mmiotrace has documented SMP race condition:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/trace/mmiotrace.rst#n135

Also playing correctly with page fault is quite a challenge. I'm trying 
to find a simpler/easier solution :)

Thanks for Qemu tip! I'll take a look


On Wed, Jan 31 2024 at 15:20:09 -06:00:00, Bjorn Helgaas 
<helgaas@kernel.org> wrote:
> On Wed, Jan 31, 2024 at 08:42:18PM +0000, nowicki@posteo.net wrote:
>>  Hello,
>> 
>>  I'm trying to implement a fake PCIe device and I'm looking for 
>> guidance (by
>>  fake I mean fully software device).
>> 
>>  So far I implemented:
>>  - fake PCIe bus with custom fake pci_ops.read & pci_ops.write 
>> functions
>>  - fake PCIe switch
>>  - fake PCIe endpoint
>> 
>>  Fake devices have implemented PCIe registers and are visible in 
>> user space
>>  via lspci tool.
>>  Registers can be edited via setpci tool.
>> 
>>  Now I'm looking for a way to implement BAR regions with custom 
>> memory
>>  handlers. Is it even possible?
>>  Basically I'd like to capture each MemoryWrite & MemoryRead 
>> targeted for
>>  PCIe endpoint's BAR region and emulate NVMe registers.
>> 
>>  I'm in dead-end right now and I'm seeing only two options:
>>  - generate page faults on every access to fake BAR region and 
>> execute fake
>>  PCIe endpoint's callbacks - similar/the same as mmiotrace
>>  - periodically scan fake BAR region for any changes
>> 
>>  Both solutions have drawbacks.
>>  Is there other way to implement fake BAR region?
> 
> Sounds kind of cool and potentially useful to build kernel test tools.
> 
> Is the page fault on access option a problem because you want better
> performance?  I assume you really *want* to know about every write and
> possibly even every read, so a page fault seems like the way to do
> that.
> 
> Maybe qemu would have some ideas?  I assume it implements some similar
> things.
> 
> Bjorn