Re: [PATCH v4] x86: add etc/phys-bits fw_cfg file

[Paolo Bonzini]

On Thu, Sep 22, 2022 at 2:21 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
> No.  This will basically inform the guest that host-phys-bits has been
> enabled (and pass the number of bits).  So the firmware can make use of
> the available address space instead of trying to be as conservative as
> possible to avoid going beyond the (unknown) limit.

Intel processors that are not extremely old have host-phys-bits equal
to 39, 46 or 52. Older processors that had 36, in all likelihood,
didn't have IOMMUs (so no big 64-bit BARs).

AMD processors have had 48 for a while, though older consumer processors had 40.

QEMU has always used 40, though many downstream packages (IIRC RHEL
and Ubuntu) just use host-phys-bits = true when using KVM.

Would it work to:

1) set host-phys-bits to true on new machine types when not using TCG
(i.e. KVM / HVF / WHPX)

2) in the firmware treat 40 as if it were 39, to support old machine types?

Paolo