Re: [PATCH 1/1] Skip flatview_simplify() for cpu vendor zhaoxin

[Paolo Bonzini]

On 21/10/20 00:44, Alex Williamson wrote:
> Do we necessarily need a memory map ioctl for this or could it be the
> QEMU code that compares the old and new maps to trigger map and unmap
> ioctls?  For example (aiui) our race is that if we have contiguous
> memory regions A and B and flatview_simplify() tries to expand A and
> delete B we'll see a series of listener notifications deleting A and B
> and adding A'.  But the vfio QEMU code could parse the memory map to
> determine that old A + B is functionally equivalent to A' and do
> nothing.

I think the issue is a bit different, and in fact there are two sides of
the same issue.  Say you have A (large) and it is replaced by A'
(smaller) + B, then:

* the first part of A disappears for a moment before A' appears.  This
is something that QEMU can work around, by not doing anything

* the second part of A disappears for a moment before B appears.  This
is the root API issue and not something that QEMU can work around; and
in fact it is not even fixed by removing flatview_simplify.

Felix, did you identify the regions whose simplification causes the bug?
 Is this RAM (for example due to the PAM registers) or something else?

Paolo