Am 28.11.2017 um 11:53 schrieb Jan Beulich:
On 28.11.17 at 11:17, <christian.koenig@xxxxxxx> wrote:
Am 28.11.2017 um 10:46 schrieb Jan Beulich:
On 28.11.17 at 10:12, <christian.koenig@xxxxxxx> wrote:
In theory the BIOS would search for address space and won't find
anything, so the hotplug operation should fail even before it reaches
the kernel in the first place.
How would the BIOS know what the OS does or plans to do?
As far as I know the ACPI BIOS should work directly with the register
content.
So when we update the register content to enable the MMIO decoding the
BIOS should know that as well.
I'm afraid I don't follow: During memory hotplug, surely you don't
expect the BIOS to do a PCI bus scan? Plus even if it did, it would
be racy - some device could, at this very moment, have memory
decoding disabled, just for the OS to re-enable it a millisecond
later. Yet looking at BAR values is meaningless when memory
decode of a device is disabled.
No, sorry you misunderstood me. The PCI bus is not even involved here.
In AMD Family CPUs you have four main types of address space routed by
the NB:
1. Memory space targeting system DRAM.
2. Memory space targeting IO (MMIO).
3. IO space.
4. Configuration space.
See section "2.8.2 NB Routing" in the BIOS and Kernel Developer’s Guide
(https://support.amd.com/TechDocs/49125_15h_Models_30h-3Fh_BKDG.pdf).
Long story short you have fix addresses for configuration and legacy IO
(VGA for example) and then configurable memory space for DRAM and MMIO.
What the ACPI BIOS does (or at least should do) is taking a look at the
registers to find space during memory hotplug.
Now in theory the MMIO space should be configurable by similar ACPI BIOS
functions, but unfortunately most BIOS doesn't enable that function
because it can break some older Windows versions.
So what we do here is just what the BIOS should have provided in the
first place.
I think
it's the other way around - the OS needs to avoid using any regions
for MMIO which are marked as hotpluggable in SRAT.
I was under the impression that this is exactly what
acpi_numa_memory_affinity_init() does.
Perhaps, except that (when I last looked) insufficient state is
(was) being recorded to have that information readily available
at the time MMIO space above 4Gb needs to be allocated for
some device.
That was also my concern, but in the most recent version I'm
intentionally doing this fixup very late after all the PCI setup is
already done.
This way the extra address space is only available for devices which are
added by PCI hotplug or for resizing BARs on the fly (which is the use
case I'm interested in).
Since there is
no vNUMA yet for Xen Dom0, that would need special handling.
I think that the problem is rather that SRAT is NUMA specific and if I'm
not totally mistaken the content is ignored when NUMA support isn't
compiled into the kernel.
When Xen steals some memory from Dom0 by hocking up itself into the e820
call then I would say the cleanest way is to report this memory in e820
as reserved as well. But take that with a grain of salt, I'm seriously
not a Xen expert.
The E820 handling in PV Linux is all fake anyway - there's a single
chunk of memory given to a PV guest (including Dom0), contiguous
in what PV guests know as "physical address space" (not to be
mixed up with "machine address space", which is where MMIO
needs to be allocated from). Xen code in the kernel then mimics
an E820 matching the host one, moving around pieces of memory
in physical address space if necessary.
Good to know.
Since Dom0 knows the machine E820, MMIO allocation shouldn't
need to be much different there from the non-Xen case.
Yes, completely agree.
I think even if we don't do MMIO allocation with this fixup letting the
kernel in Dom0 know which memory/address space regions are in use is
still a good idea.
Otherwise we will run into exactly the same problem when we do the MMIO
allocation with an ACPI method and that is certainly going to come in
the next BIOS generations because Microsoft is pushing for it.
Regards,
Christian.
Jan
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