Re: [Qemu-devel] [PATCH v3] hw/arm/virt: Add high MMIO PCI region

[Pavel Fedin]

 Hi! I have done an additional study...

> (1) We should confirm whether this really is a guest kernel
> bug (as opposed to the device tree QEMU emits not being
> in spec)
> (2) If it is a kernel bug, submit a patch to fix it

 It is a bug, but not major bug. The bug is only that the kernel tries to map 
the device anyway, despite it takes wrong, truncated physical addresses. But, 
if we fix it, then the only option for the kernel is not to try to use this 
device at all. Because, in general case, this would mean that the device is 
outside of usable address space, and we cannot access it, therefore cannot 
drive it properly.
 Therefore, the non-LPAE-enabled kernel will not be able to use PCI controller 
with high MMIO anyway.

> (3) Consider a workaround for older guests anyway. The
> scope of that workaround would depend on exactly which
> guests are affected, which is presumably something we
> figured out during step (1).

 The behavior which i explained above causes boot problems if our configuration 
assumes that we boot off emulated PCI device. Because PCI controller becomes 
unusable. Therefore, if we want to stay compatible with such guests (which 
would be indeed old), we need an option which allows to turn off high MMIO 
region.
 Now the question is: what do we want as default on 32-bit virt? On 64 bits the 
default would be obviously "ON", what about 32 bits?

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia