Re: [RFC PATCH 0/5] support CPU hot-unplug

["Laszlo Ersek" <lersek@redhat.com>]

Hi Ankur,

On 12/08/20 06:34, Ankur Arora wrote:
> [ Resending to the correct edk2 alias this time. ]
> 
> Hi,
> 
> This series adds support for CPU hot-unplug with OVMF.
> 
> Please see this in conjunction with the QEMU v2 series posted here: 
>   https://lore.kernel.org/qemu-devel/20201207140739.3829993-1-imammedo@redhat.com/
> 
> In particular, would be glad for comments on Patch 4, specifically
> where we should be ejecting the CPU. 
> 
> Right now the ejection happens in UnplugCpus() (called from
> CpuHotplugMmi()):
>   +    QemuCpuhpWriteCpuSelector (mMmCpuIo, RemoveApicId);
>   +    QemuCpuhpWriteCpuStatus (mMmCpuIo, QEMU_CPUHP_STAT_EJECTED);
> 
> That is way too early -- given that the actual unplug will happen
> in SmmCpuUpdate() and given that the BSPHandler() would have waited
> for the APs multiple times before then.
> 
> Another possibility is that the actual ejection be deferred to the
> _EJ0 method after the return from the SMI. But, that seems like a
> hack. Additionally, Igor points out here that this approach has problems:
>   https://lore.kernel.org/qemu-devel/20201204170939.1815522-1-imammedo@redhat.com/
> 
> Please review.

thanks for the patches; I'm confirming I've got them.

I'll need a non-trivial amount of time before I come to these patches
(and to the QEMU patches posted by Igor).

I'm working very busily on
<https://bugzilla.tianocore.org/show_bug.cgi?id=3097> and my brain is
full of other stuff.

We had the reverse situation earlier this year, I think, when -- in
relation to hotplug -- Igor was occupied with a more pressing QEMU
development (NUMA I think?), for a significant amount of time.

What's important is that I want to do both Igor's patches and your
patches *justice*, with my review, and at this time I just cannot sit
down with them alone for a day. These patches deserve a deep looking-at,
rather than a skim, and I cannot afford the former at the moment. I
prefer doing a (hopefully) thorough review, later, to rushing a review now.

I hope that's tolerable.

Thank you,
Laszlo