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Re: [Xen-devel] [PATCH] x86/libxl: choose a sane default for HAP
- To: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 5 Sep 2019 11:00:36 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, TimDeegan <tim@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Paul Durrant <Paul.Durrant@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 05 Sep 2019 10:00:45 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 05/09/2019 10:52, 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?
We could, but I would object to doing so.
It is not sensible behaviour to have, because the only thing it does is
hide (mis)configuration issues.
>
>> --- a/xen/include/public/sysctl.h
>> +++ b/xen/include/public/sysctl.h
>> @@ -90,6 +90,10 @@ struct xen_sysctl_tbuf_op {
>> /* (x86) The platform supports direct access to I/O devices with IOMMU. */
>> #define _XEN_SYSCTL_PHYSCAP_directio 2
>> #define XEN_SYSCTL_PHYSCAP_directio (1u<<_XEN_SYSCTL_PHYSCAP_directio)
>> +/* (x86) The platform supports Hardware Assisted Paging. */
>> +#define _XEN_SYSCTL_PHYSCAP_hap 3
>> +#define XEN_SYSCTL_PHYSCAP_hap (1u<<_XEN_SYSCTL_PHYSCAP_hap)
> I think this bit wants to be universal (i.e. "(x86)" dropped), and
> be set unconditionally on Arm. Irrespective of the question regarding
> an alternative solution I think it is quite sensible to expose
> availability of HAP to the tools. In fact I think "xl info" should
> show this alongside other properties.
I agree. While terms like HVM and HAP may have come about in the x86
world, they are deliberately vendor and technology neutral.
ARM *should* be claim to be, and behave as, HVM-only HAP-only system,
per this nomenclature.
~Andrew
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