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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.09.15 at 14:19, <roger.pau@xxxxxxxxxx> wrote:
> El 30/09/15 a les 13.54, Jan Beulich ha escrit:
>>>>> On 30.09.15 at 13:37, <roger.pau@xxxxxxxxxx> wrote:
>>>     /*
>>>      * Using VCPU_HVM_MODE_64B implies that the vCPU is launched
>>>      * directly in long mode, so the type of the cached part
>>>      * of the TR register is set to describe a 64-bit TSS (Busy).
>>>      * The cached part of the CS register will also have the L bit
>>>      * set (64-bit code segment).
>> 
>> I'd leave out mentioning TR here (or else it'll be odd not to mention
>> e.g. LDTR too). Perhaps just "..., so the cached parts of the segment
>> registers get set to match that environment"?
> 
> That sounds fine. I'm going to update the patch and the FreeBSD part in
> order to test it. Since we also spoke about adding sanity checks, I
> wonder whether I should add those checks now, or leave them for a later
> patch. IMHO those checks are only useful for developers.

Fundamentally, checks done here should include everything that
would otherwise lead to the domain getting killed due to failed
VMENTRY. I.e. both sets below may need extending.

> For VCPU_HVM_MODE_32B:
>  - rIP within CS limit.
>  - Check that CS.DPL == SS.DPL.
>  - rSP within SS limit.
> 
> TBH I don't think we should enforce the last two checks, starting with
> an invalid stack should be fine as long as the user knows it. Maybe
> print a warning/debug message in this case?

I wouldn't check ESP at all. As to the two DPLs to, I don't think you
could launch a guest with these disagreeing.

> For VCPU_HVM_MODE_64B:
>  - Check that cr0 has paging enabled.
>  - Check that cr4 has pae enabled.
>  - Check that efer has the LMA/LME bits set.
> 
> Those should be always enforced for long mode.

Agreed, plus RIP being canonical.

Jan


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