On Thu, Sep 22, 2022 at 2:21 PM Gerd Hoffmann <kraxel@xxxxxxxxxx> 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
