RE: [PATCH] arm64: turn off xgene branch prediction while in kernel space

[David Laight]

From: Will Deacon
> Sent: 24 January 2018 16:43
> On Wed, Jan 24, 2018 at 11:35:03AM -0500, Mark Salter wrote:
> > On Wed, 2018-01-24 at 10:58 +0000, Marc Zyngier wrote:
> > > Khuong,
> > >
> > > On 24/01/18 02:13, Khuong Dinh wrote:
> > > > Aliasing attacks against CPU branch predictors can allow an attacker to
> > > > redirect speculative control flow on some CPUs and potentially divulge
> > > > information from one context to another.
> > > >
> > > > This patch only supports for XGene processors.
...
> > > Why isn't this using the infrastructure that is already in place?
> >
> > That infrastructure relies on a cpu-specific flush of the branch
> > predictor. XGene does not have the ability to flush the branch
> > predictor. It can only turn it on or off.
> 
> So how does this patch protect one user application from another? Sounds
> like you need to turn the thing off at boot and leave it that way, or find
> a sequence of branch instructions to effectively do the invalidation.

What sort of performance penalty does this give?
I can imagine it is significant.

Attempting to flush a branch predictor is also likely to be very slow.

	David