[RFC PATCH 2/2] qemu: Add support for max physical address size

[Daniel P. Berrangé]

On Thu, Oct 29, 2020 at 03:55:42PM +0000, Dario Faggioli wrote:
> This patch maps /domain/cpu/maxphysaddr into -cpu parameters:
> 
>   - <maxphysaddr mode='passthrough'/> becomes host-phys-bits=on
>   - <maxphysaddr mode='emualte' bits='42'/> becomes phys-bits=42
> 
> Passthrough mode can only be used if the chosen CPU model is
> 'host-passthrough'.
> 
> The feature is available since QEMU 2.7.0.

I wonder if we need to care about doing any kind of host compatibility
checking when these are specified.

QEMU historically hasn't checked compatibility, meaning you could
configure bits=48 on a host CPU that only supports bits=40. Things
would seem to work unless you gave a guest RAM size that needed
soo many bits.

QEMU is reluctant to change things due to back compat issues with
existing deployments. Libvirt wouldn't have that issue though, as
we would only need to enforce it when users actually gave the
<maxphysaddr> element, which is opt-in and won't be present in
existing guests.


Regards,
Daniel
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