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

[Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>]

On Mon, Jun 27, 2016 at 3:09 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
>> > 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.

Thanks a lot! I will give this a shot... I'm thinking I've covered the
other bases so maybe this will finally make it work as expected!

>> 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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