Re: [PATCH 1/3] x86/mem_sharing: option to skip populating special pages during fork

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Fri, Mar 25, 2022, 6:59 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:

On Tue, Mar 22, 2022 at 01:41:37PM -0400, Tamas K Lengyel wrote:
> Add option to the fork memop to skip populating the fork with special pages.
> These special pages are only necessary when setting up forks to be fully
> functional with a toolstack. For short-lived forks where no toolstack is active
> these pages are uneccesary.

Replying here because there's no cover letter AFAICT.

For this kind of performance related changes it would be better if you
could provide some figures about the performance impact. It would help
if we knew whether avoiding mapping the vAPIC page means you can
create 0.1% more forks per-minute or 20%.

If you really want to speed up the forking path it might be good to start
by perf sampling Xen in order to find the bottlenecks?

Sure but for experiment systems I don't think its necessary to collect that data.

There is also a non-performance reason why we want to keep special pages from being populated, in cases we really want the forks physmap to start empty for better control over its state. There was already a case where having special pages mapped in ended up triggering unexpected Xen behaviors leading to chain of events not easy to follow. For example if page 0 gets brought in while the vCPU is being created it ends up as a misconfigured ept entry if nested virtualization is enabled. That leads to ept misconfiguration exits instead of epf faults. Simply enforcing no entry in the physmap until forking is complete eliminates the chance of something like that happening again and makes reasoning about the VM's behavior from the start easier.

Tamas