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

[Paolo Bonzini <pbonzini@redhat.com>]

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Il gio 22 set 2022, 22:33 Gerd Hoffmann <kraxel@redhat.com> ha scritto:

> On Thu, Sep 22, 2022 at 02:38:02PM +0200, Paolo Bonzini wrote:
> > 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.
>
> How reliable is the vendorid?
>

Pretty reliable. In principle it can be changed, but there's no good reason
to do it (especially in a long lived VM) and it requires manual command
line intervention.


> Given newer processors have more than 40 and for older ones we know
> the possible values for the two relevant x86 vendors we could do
> something along the lines of:
>
>    phys-bits >= 41                   -> valid
>    phys-bits == 40    + AuthenticAMD -> valid
>    phys-bits == 36,39 + GenuineIntel -> valid
>    everything else                   -> invalid
>
> Does that look sensible to you?
>

Yes, it does! Is phys-bits == 36 the same as invalid? If so that's even one
fewer special case to handle.

Paolo


> take care,
>   Gerd
>
>

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