On Thu, Jun 20, 2024 at 02:48:03PM GMT, Kevin O'Connor wrote:
> On Thu, Jun 20, 2024 at 07:08:38PM +0100, John Levon wrote:
> > On Thu, Jun 20, 2024 at 01:29:11PM -0400, Kevin O'Connor wrote:
> >
> > > I'm leery of moving this heuristic to 64G of ram. I can understand
> > > the logic of >4G of ram indicating support for >4G pci. However, it
> > > seems strange to me that there would be guests with 50G of ram that
> > > can't handle >4G pci, but not similar guests with 70G of ram.. It
> >
> > Such a guest cannot possibly address that much, so it doesn't seem at all
> > unreasonable to require a config change there. That is, by definition there
> > can't be a workload in the OS that's relying on 70G of RAM.
>
> Thanks. I missed that PAE is limited to 36bits (64GB).
I had assumed that, but it's not. That was the case in all 32-bit
processors with PAE support.
But 64-bit processors have the same physical address space limit in both
long mode and pae paging mode. For early intel processors that happened
to be 64GB too.
> Is the problem that SeaBIOS created PCI mappings >4G or is the problem
> that SeaBIOS created PCI mappings >64G?
Good question. Older linux kernels have known problems with mappings
above 64TB (aka 46 phys-bits). This is addressed by commit a6ed6b701f0a
("limit address space used for pci devices.").
John, can you check with your guests? Try reduce pci_mem64_top and see
if things start working at some point.
take care,
Gerd
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