Re: [Xen-devel] Elaboration of "Question about sharing spinlock_t among VMs in Xen"

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> > One other thing you should be aware of: as soon as one of your guests
> > has only one vcpu it will drop the "lock" prefixes for updates of the
> > lock word. So there will be a chance of races just because one or both
> > guests are thinking no other cpu can access the lock word concurrently.
> 
> Now that is an interesting point! I am indeed using 1 vcpu for each
> domain right now. Does it automatically drop lock if it detects one
> vcpu when booting? Or is this set at compile time? Shouldn't setting

It does patching of the binary during bootup time.

You can disable the patching by using noreplace-smp on Linux command line.


> SMP to y regardless of core/vcpu count keep the SMP spinlock
> implementation? I definitely did not think about this -- It was
> compiled with one vcpu so if its done at compile time this could be
> the issue. I doubt its done at boot but if so I would presume there is
> a way to disable this?
> 
> Below is the config file grepped for "SMP".
> CONFIG_X86_64_SMP=y
> CONFIG_GENERIC_SMP_IDLE_THREAD=y
> CONFIG_SMP=y
> # CONFIG_X86_VSMP is not set
> # CONFIG_MAXSMP is not set
> CONFIG_PM_SLEEP_SMP=y
> 
> See anything problematic? Seems PV spinlocks is not set, and SMP is
> enabled... or is something else required to prevent stripping the
> spinlocks? Also not sure if any of the set SPIN config items could
> mess with this. If this is done at boot, a point in the direction for
> preventing this would be appreciated!
> 
> Regards,
> Dagaen Golomb
> Ph.D Student, University of Pennsylvania

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