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Re: [PATCH v4] x86: add etc/phys-bits fw_cfg file
From: |
Paolo Bonzini |
Subject: |
Re: [PATCH v4] x86: add etc/phys-bits fw_cfg file |
Date: |
Thu, 22 Sep 2022 14:38:02 +0200 |
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