On Tue, 07 Sep 2021 19:25:23 +0100,
Peter Maydell [off-list ref] wrote:

On Tue, 7 Sept 2021 at 18:10, Marc Zyngier [off-list ref] wrote:

quoted

Hi Peter,

On Tue, 07 Sep 2021 13:51:13 +0100,
Peter Maydell [off-list ref] wrote:

quoted

On Sun, 22 Aug 2021 at 15:45, Marc Zyngier [off-list ref] wrote:

quoted

The documentation for the 'highmem' option indicates that it controls
the placement of both devices and RAM. The actual behaviour of QEMU
seems to be that RAM is allowed to go beyond the 4GiB limit, and
that only devices are constraint by this option.

Align the documentation with the actual behaviour.

I think it would be better to align the behaviour with the documentation.

The intent of 'highmem' is to allow a configuration for use with guests
that can't address more than 32 bits (originally, 32-bit guests without
LPAE support compiled in). It seems like a bug that we allow the user
to specify more RAM than will fit into that 32-bit range. We should
instead make QEMU exit with an error if the user tries to specify
both highmem=off and a memory size that's too big to fit.

I'm happy to address this if you are OK with the change in user
visible behaviour.

However, I am still struggling with my original goal, which is to
allow QEMU to create a usable KVM_based VM on systems with a small IPA
space (36 bits on the system I have). What would an acceptable way to
convey this to the code that deals with the virt memory map so that it
falls back to something that actually works?

Hmm, so at the moment we can either do "fits in 32 bits" or
"assumes at least 40 bits" but not 36 ?

Exactly. I have the gut feeling that we need a 'gpa_bits' option that
would limit the guest physical range and generalise highmem. High IO
ranges would simply not be available if the GPA range isn't big
enough.

	M.

-- 
Without deviation from the norm, progress is not possible.