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


  • To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Thu, 5 Sep 2019 12:44:16 +0100
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, "Tim \(Xen.org\)" <tim@xxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Paul Durrant <Paul.Durrant@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxx>, Anthony Perard <anthony.perard@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 05 Sep 2019 11:44:47 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 9/5/19 12:01 PM, Roger Pau Monné wrote:
> 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.

Right; I wasn't trying to contradict you so much as "weigh in" (and
basically agree with you).

 -George

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