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

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> 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”.  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.

 -George

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