Re: [PATCH] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

[Sean Christopherson]

On Thu, Feb 06, 2020 at 11:37:04AM -0800, Andy Lutomirski wrote:
> 
> > On Feb 6, 2020, at 8:46 AM, Luck, Tony <tony.luck@xxxxxxxxx> wrote:
> > 
> > ïOn Wed, Feb 05, 2020 at 05:18:23PM -0800, Andy Lutomirski wrote:
> >>> On Wed, Feb 5, 2020 at 4:49 PM Luck, Tony <tony.luck@xxxxxxxxx> wrote:
> >>> 
> >>> In a context switch from a task that is detecting split locks
> >>> to one that is not (or vice versa) we need to update the TEST_CTRL
> >>> MSR. Currently this is done with the common sequence:
> >>>        read the MSR
> >>>        flip the bit
> >>>        write the MSR
> >>> in order to avoid changing the value of any reserved bits in the MSR.
> >>> 
> >>> Cache the value of the TEST_CTRL MSR when we read it during initialization
> >>> so we can avoid an expensive RDMSR instruction during context switch.
> >> 
> >> If something else that is per-cpu-ish gets added to the MSR in the
> >> future, I will personally make fun of you for not making this percpu.
> > 
> > Xiaoyao Li has posted a version using a percpu cache value:
> > 
> > https://lore.kernel.org/r/20200206070412.17400-4-xiaoyao.li@xxxxxxxxx
> > 
> > So take that if it makes you happier.  My patch only used the
> > cached value to store the state of the reserved bits in the MSR
> > and assumed those are the same for all cores.
> > 
> > Xiaoyao Li's version updates with what was most recently written
> > on each thread (but doesn't, and can't, make use of that because we
> > know that the other thread on the core may have changed the actual
> > value in the MSR).
> > 
> > If more bits are implemented that need to be set at run time, we
> > are likely up the proverbial creek. I'll see if I can find out if
> > there are plans for that.
> > 
> 
> I suppose that this whole thing is a giant mess, especially since at least
> one bit there is per-physical-core. Sigh.
> 
> So I donât have a strong preference.

I'd prefer to go with this patch, i.e. not percpu, to remove the temptation
of incorrectly optimizing away toggling SPLIT_LOCK_DETECT.