[MODERATED] Re: [patch 4/8] MDS basics 4

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 19, 2019 at 08:09:13AM -0800, speck for Andi Kleen wrote:
> On Tue, Feb 19, 2019 at 03:07:59PM +0100, speck for Thomas Gleixner wrote:
> > On Tue, 19 Feb 2019, speck for Thomas Gleixner wrote:
> > > On Tue, 19 Feb 2019, speck for Andrew Cooper wrote:
> > > 
> > > > On 19/02/2019 12:44, speck for Thomas Gleixner wrote:
> > > > > Subject: [patch 4/8] x86/speculation/mds: Conditionaly clear CPU buffers on idle entry
> > > > > From: Thomas Gleixner <tglx@linutronix.de>
> > > > >
> > > > > Add a static key which controls the invocation of the CPU buffer clear
> > > > > mechanism on idle entry. This is independent of other MDS mitigations
> > > > > because the idle entry invocation to mitigate the potential leakage due to
> > > > > store buffer repartitioning is only necessary on SMT systems.
> > > > >
> > > > > Add the actual invocations to the different halt/mwait variants which
> > > > > covers all usage sites. mwaitx is not patched as it's not available on
> > > > > Intel CPUs.
> > > > >
> > > > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > > > 
> > > > Unfortunately, clearing is needed on the exit from idle as well as the
> > > > entry.
> > > > 
> > > > This only impacts the store buffer (MSBDS, previously PSF) because it
> > > > gets statically re-partitioned when a thread comes in and out of idle.
> > > > 
> > > > >From the point of view of the thread going idle, when going idle my half
> > > > of the store buffers get given to the other thread and potentially leak
> > > > my secrets, whereas when coming out of idle, the other threads store
> > > > buffers get split with me, potentially leaking their secrets.
> > > 
> > > Duh, indeed. Easy enough to fix.
> > 
> > Delta patch below. Stupid me even mentioned the repartioning on both sides
> > in the changelog.
> 
> It's not needed.

But will the part of the store buffer that gets repartitioned not
contain state of the other sibling?

If it can see our data after we go idle; why can't we see its data when
we come out of idle?