Re: [Xen-devel] [PATCH] x86/libxl: choose a sane default for HAP

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

On Thu, Sep 05, 2019 at 12:34:11PM +0200, George Dunlap wrote:
> 
> 
> > On Sep 5, 2019, at 11:01 AM, Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
> > 
> > On Thu, Sep 05, 2019 at 11:52:59AM +0200, Jan Beulich wrote:
> >> On 05.09.2019 11:34, Roger Pau Monne wrote:
> >>> Current libxl code will always enable Hardware Assisted Paging (HAP),
> >>> expecting that the hypervisor will fallback to shadow if HAP is not
> >>> available. With the changes to the domain builder that's not the case
> >>> any longer, and the hypervisor will raise an error if HAP is not
> >>> available instead of silently falling back to shadow.
> >> 
> >> Would it really be much more involved than the change here to restore
> >> silent defaulting to shadow?
> > 
> > But that would mean that a user having selected hap=1 on the config
> > file would get silently defaulted to shadow, which is wrong IMO.
> 
> At the libxl layer, aren’t the options tristate?  I.e., this would be “hap”, 
> “shadow”, or “not specified”?
> 
> The user needs to be able to specify “always use shadow”, “always use HAP”, 
> or “use HAP if available, otherwise use shadow”.

The "use HAP if available, otherwise use shadow" is currently only
possibly expressed by not setting the hap option in the config file.

> At the moment, leaving it empty should be “use HAP if available, otherwise 
> use shadow”; so “hap = 1” should fail if HAP is not available.

Right, this is what this patch is trying to accomplish.

I have a v2 series which fills the capabilities field for ARM and also
reports the hap capability in `xl info`.

Thanks, Roger.

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