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Re: Plugin Address Translations Inconsistent/Incorrect?

From: Aaron Lindsay
Subject: Re: Plugin Address Translations Inconsistent/Incorrect?
Date: Mon, 22 Feb 2021 15:48:46 -0500

On Feb 22 19:30, Alex Bennée wrote:
> Aaron Lindsay <aaron@os.amperecomputing.com> writes:
> > 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
> yet.

I don't think I yet understand why this is a problem. It seems to me
that the current implementation of `qemu_plugin_hwaddr_device_offset`
returns offsets which may already be ambiguous without additional
information about the underlying device/memory, and I'm not sure why
translating to full physical addresses would make that worse. It's
possible I'm not correctly interpreting your concern.

> 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.

Would the `block->mr->addr` field I mentioned above be updated in such a

> 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.

Haven't put a ton of thought into it, but what about something like this

uint64_t qemu_plugin_hwaddr_phys_addr(const struct qemu_plugin_hwaddr *haddr)
    if (haddr) {
        if (!haddr->is_io) {
            RAMBlock *block;
            ram_addr_t offset;

            block = qemu_ram_block_from_host((void *) haddr->v.ram.hostaddr, 
false, &offset);
            if (!block) {
                error_report("Bad ram pointer %"PRIx64"", 

            return block->offset + offset + block->mr->addr;
        } else {
            MemoryRegionSection *mrs = haddr->v.io.section;
            return haddr->v.io.offset + mrs->mr->addr;
    return 0;

The key differences from `qemu_plugin_hwaddr_device_offset` are using
`qemu_ram_block_from_host` directly instead of `qemu_ram_addr_from_host` (to
get a pointer to the RAMBlock), and adding `block->mr->addr` and
`mrs->mr->addr` to the returns for RAM and IO, respectively.


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