Re: [Xen-devel] Deprecating HVM_PARAM_PAE_ENABLED

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jul 09, 2018 at 05:54:44PM +0100, Andrew Cooper wrote:
> Hello,
> 
> (I feel as if I've sent this email before, but I can't find any record
> of it).
> 
> The parameter was introduced in e661d66f51 (2006) and used in Xen, but
> removed from the hypervisor in c/s 5f14a87ce (2008) when CPUID
> calculations where moved from the hypervisor into libxc.
> 
> However, the field has always been propagated in the migration stream. 
> AFAICT, the only use for the HVM_PARAM is as a function parameter to
> xc_cpuid_apply_policy(), which is a very very expensive way of passing a
> function parameter!
> 
> Another curiosity is that HVM and PVH guests treat the toplevel xl.cfg
> pae boolean differently.  HVM honour the request, while PVH blindly
> ignores it and sets it to true.  There is nothing wrong (in principle)
> with a PVH non-PAE guest, so the boolean should be honoured IMO.

Yes, this seems like a bug to me. When fixing it could you also please
amend xl.cfg(5) to add that the `pae` option is available to PVH
guests?

> A separate usability niggle is that there is no interlink between the
> toplevel pae boolean and passing some custom cpuid= configuration.  The
> latter appears to take priority but only due to the order in which libxl
> processes the data.
> 
> Does anyone see any reason to keep HVM_PARAM_PAE_ENABLED around?

Given the argument above, no.

Thanks, Roger.

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