Re: HVM guest only bring up a single vCPU

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 27/08/2021 10:26, Jan Beulich wrote:
> On 26.08.2021 23:00, Julien Grall wrote:
> > Digging down, Linux will set smp_num_siblings to 0 (via
> > detect_ht_early()) and as a result will skip all the CPUs. The value is
> > retrieve from a CPUID leaf. So it sounds like we don't set the leaft
> > correctly.
> Xen leaves leaf 1 EBX[23:16] untouched from what the tool stack
> passes. The tool stack doubles the value coming from hardware
> (or qemu in your case), unless the result would overflow. Hence
> it would look to me as if the value coming from qemu has got to
> be zero. Which is perfectly fine if HTT is off, just that
> libxenguest isn't prepared for this. 
> Could you see whether the
> patch below helps (making our hack even hackier)?
It helps. The Linux HVM domain is now able to bring up all the CPUs.

> Jan
>
> libxenguest/x86: ensure CPUID[1].EBX[32:16] is non-zero for HVM
>
> We unconditionally set HTT, so merely doubling the value read from
> hardware isn't going to be correct if that value is zero.
>
> Reported-by: Julien Grall <julien@xxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Feel free to add my tested-by for the patch.

Cheers,

[1] https://pastebin.com/WvaXiXuL

--
Julien Grall