[PATCH v5 08/14] KVM: arm64: Protect stage-2 traversal with RCU

[Ricardo Koller]

On Wed, Nov 09, 2022 at 11:55:31PM +0000, Oliver Upton wrote:
> On Wed, Nov 09, 2022 at 09:53:45PM +0000, Sean Christopherson wrote:
> > On Mon, Nov 07, 2022, Oliver Upton wrote:
> > > Use RCU to safely walk the stage-2 page tables in parallel. Acquire and
> > > release the RCU read lock when traversing the page tables. Defer the
> > > freeing of table memory to an RCU callback. Indirect the calls into RCU
> > > and provide stubs for hypervisor code, as RCU is not available in such a
> > > context.
> > > 
> > > The RCU protection doesn't amount to much at the moment, as readers are
> > > already protected by the read-write lock (all walkers that free table
> > > memory take the write lock). Nonetheless, a subsequent change will
> > > futher relax the locking requirements around the stage-2 MMU, thereby
> > > depending on RCU.
> > 
> > Two somewhat off-topic questions (because I'm curious):
> 
> Worth asking!
> 
> >  1. Are there plans to enable "fast" page faults on ARM?  E.g. to fixup access
> >     faults (handle_access_fault()) and/or write-protection faults without acquiring
> >     mmu_lock?
> 
> I don't have any plans personally.
> 
> OTOH, adding support for read-side access faults is trivial, I just
> didn't give it much thought as most large-scale implementations have
> FEAT_HAFDBS (hardware access flag management).

WDYT of permission relaxation (write-protection faults) on the fast
path?

The benefits won't be as good as in x86 due to the required TLBI, but
may be worth it due to not dealing with the mmu lock and avoiding some
of the context save/restore.  Note that unlike x86, in ARM the TLB entry
related to a protection fault needs to be flushed.

> 
> >  2. If the answer to (1) is "yes!", what's the plan to protect the lockless walks
> >     for the RCU-less hypervisor code?
> 
> If/when we are worried about fault serialization in the lowvisor I was
> thinking something along the lines of disabling interrupts and using
> IPIs as barriers before freeing removed table memory, crudely giving the
> same protection as RCU.
> 
> --
> Thanks,
> Oliver