Re: [Xen-devel] [PATCH v6 24/29] xen/x86: allow HVM guests to use hypercalls to bring up vCPUs

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/09/15 08:09, Jan Beulich wrote:
>
>>>> +    uint32_t cs_base;
>>>> +    uint32_t ds_base;
>>>> +    uint32_t ss_base;
>>> I continue to question why we have DS here, but not ES (and maybe
>>> FS and GS too). I.e. either just CS and SS (which are architecturally
>>> required) or at least all four traditional x86 segment registers. And
>>> you're also clearly not targeting minimal state, or else there likely
>>> wouldn't be a need for e.g. R8-R15 in the 64-bit variant.
>> I'm fine with removing r8-15. Regarding the segment selectors, I don't
>> have a problem with only allowing CS and SS to be set, or all of them
>> including FS and GS. But I would like to get a consensus on this, we
>> have already gone back and forth several times regarding how this
>> structure should look like, and TBH, I was hoping that this was the last
>> time.
> Was there back and forth? I only recall always having asked for
> consistency here, just like spelled out above.
>
>> Andrew, Jan, what would you prefer, either DS is removed or ES, FS and
>> GS are also added?
> I voiced my opinion. Andrew?

DS clearly needs initialising to provide a sane environment in the newly
running vcpu.  Expecting %cs or %ss overrides until a new GDT is loaded
is unreasonable IMO.

Therefore, we are back to the question of whether to provide all segment
registers, or specify a flat layout without specific selector values.  I
would prefer the former to the latter.

~Andrew

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