On Thu, Jan 25, 2018 at 08:54:13PM +0100, Christoffer Dall wrote:
> On Tue, Jan 23, 2018 at 04:04:40PM +0000, Dave Martin wrote:
> > On Fri, Jan 12, 2018 at 01:07:32PM +0100, Christoffer Dall wrote:
> > > We are about to defer saving and restoring some groups of system
> > > registers to vcpu_put and vcpu_load on supported systems.  This means
> > > that we need some infrastructure to access system registes which
> > > supports either accessing the memory backing of the register or directly
> > > accessing the system registers, depending on the state of the system
> > > when we access the register.
> > > 
> > > We do this by defining a set of read/write accessors for each system
> > > register, and letting each system register be defined as "immediate" or
> > > "deferrable".  Immediate registers are always saved/restored in the
> > > world-switch path, but deferrable registers are only saved/restored in
> > > vcpu_put/vcpu_load when supported and sysregs_loaded_on_cpu will be set
> > > in that case.
> > > 
> > > Not that we don't use the deferred mechanism yet in this patch, but only
> > > introduce infrastructure.  This is to improve convenience of review in
> > > the subsequent patches where it is clear which registers become
> > > deferred.
> > 
> > Might this table-driven approach result in a lot of branch mispredicts,
> > particularly across load/put boundaries?
> > 
> > If we were to move the whole construct to a header, then it could get
> > constant-folded at the call site down to the individual reg accessed,
> > say:
> > 
> >     if (sys_regs_loaded)
> >             read_sysreg_s(TPIDR_EL0);
> >     else
> >             __vcpu_sys_reg(v, TPIDR_EL0);
> > 
> > Where multiple regs are accessed close to each other, the compiler
> > may be able to specialise the whole sequence for the loaded and !loaded
> > cases so that there is only one conditional branch.
> > 
> That's an interesting thing to consider indeed.  I wasn't really sure
> how to put this in a header file which wouldn't look overly bloated for
> inclusion elsewhere, so we ended up with this.
> I don't think the alternative suggestion that I discused with Julien on
> this patch changes this much, but since you've had a look at this, I'm
> curious which one of the two (lookup table vs. giant switch) you prefer?

The giant switch approach has the advantage that it is likely to be
folded down to a single case when the switch control expression is
const-foldable; the flipside is that when that fails the entire
switch would be inlined.

> > The individual accessor functions also become unnecessary in this case,
> > because we wouldn't need to derive function pointers from them any
> > more.
> > 
> > I don't know how performance would compare in practice though.
> I don't know either.  But I will say that the whole idea behind put/load
> is that you do this rarely, and going to userspace from KVM is
> notriously expensive, also on x86.

I guess that makes sense.  I'm still a bit hazy on the big picture
for KVM.

> > I'm also assuming that all calls to these accessors are const-foldable.
> > If not, relying on inlining would bloat the generated code a lot.
> We have places where this is not the cae, access_vm_reg() for example.
> But if we really, really, wanted to, we could rewrite that to have a
> function for each register, but that's pretty horrid on its own.

That might not be too bad if there is only one giant inline expansion
and the rest are folded down.

I guess this is something to revisit _if_ we suspect a performance
bottleneck later on.

For now, I was lacking some understanding regarding how this code gets
run, so I was guessing about potential issues rather then proven

As you might guess, I'm still at the "stupid questions" stage for
this series :)


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