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

[Bjorn Helgaas <helgaas@kernel.org>]

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