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Re: Plugin Address Translations Inconsistent/Incorrect?

From: Alex Bennée
Subject: Re: Plugin Address Translations Inconsistent/Incorrect?
Date: Mon, 22 Feb 2021 19:30:04 +0000
User-agent: mu4e 1.5.8; emacs 28.0.50

Aaron Lindsay <aaron@os.amperecomputing.com> writes:

> Hello,
> I've been doing some more work with plugins and found something I didn't
> expect with regards to address translation.
> If I call (inside a memory callback):
> `uint64_t pa = qemu_plugin_hwaddr_device_offset(hwaddr);`
> I see that `pa` takes the value 0xe0e58760. If, however, I plumb
> `cpu_get_phys_page_debug` through to the plugin interface and call it
> like:
> `pa = cpu_get_phys_page_debug(current_cpu, va);`
> I see it takes the value 0x120e58760.
> I notice that 0x120e58760-0xe0e58760 is exactly one gigabyte, which is
> also the offset of the beginning of RAM for the 'virt' AArch64 machine
> I'm using. Furthermore, I see the name of the plugin function includes
> "device_offset", so perhaps this discrepancy is by design. However, it
> seems awkward to not be able to get a true physical address.

It certainly is by design. The comment for the helper states:

   * The following additional queries can be run on the hwaddr structure
   * to return information about it. For non-IO accesses the device
   * offset will be into the appropriate block of RAM.

> I've done some digging and found that inside `qemu_ram_addr_from_host`
> (called by `qemu_plugin_hwaddr_device_offset`), `block->mr->addr`
> appears to hold the offset of the beginning of RAM. 
> Do you think it would be reasonable to modify
> `qemu_plugin_hwaddr_device_offset` to add the beginning of the RAM block
> or otherwise return the true physical address (or at least expose a way
> to find the beginning of it through the plugin interface)?

Well the problem here is what is the address map? For example if you
have a secure block of RAM you might have two physical addresses which
are the same. That said with the current qemu_plugin_hwaddr_device_name
helper both will get reported as "RAM" so maybe it's not that helpful

I also worry about what happens if devices get moved around. Do you end
up with aliasing of address space have a remap of the HW.

That said I think we could add an additional helper to translate a
hwaddr to a global address space address. I'm open to suggestions of the
best way to structure this.

> Thanks!
> -Aaron

Alex Bennée

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