Re: [Xen-devel] [PATCH RFC v1 40/74] xen/x86: report domain id on cpuid

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/01/18 10:27, Jan Beulich wrote:
>>>> On 04.01.18 at 14:05, <wei.liu2@xxxxxxxxxx> wrote:
>> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>>
>> Use the ebx register of the hypervisor leaf 1. The eax register on
>> this leaf is already used to report the Xen major and minor versions.
> The rationale for doing this is missing. Iirc in past discussions the
> opinion was voiced (more than once, and iirc by Andrew any maybe
> others) that a domain in general shouldn't be told about its domain
> ID. Otherwise I also can't see why we don't have a hypercall for
> this, and e.g. XTF needs to go through hoops to figure it out. Are
> those arguments (which I don't recall) not applicable anymore?
>
> In the Amazon shim patches thread handing out the domain ID by
> command line option was suggested as an alternative, which then
> wouldn't affect other (non-shim) domains, or the client of the shim.

A guests domid is unconditionally always available in xenstore, and is a
necessary part of any PV communication.

Like it or not, domid is part of the guests view of the Xen ABI. 
Therefore, making it easily accessible is the best course of action
(especially as pv-shim deliberately doesn't interpose on the xenstore ring).

~Andrew

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