Re: [RFC PATCH 5/5] KVM: PPC: Take the SRCU lock around memslot use

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Aug 06, 2012 at 08:08:16PM +1000, Paul Mackerras wrote:
> The generic KVM code uses SRCU (sleeping RCU) to protect accesses
> to the memslots data structures against updates due to userspace
> adding, modifying or removing memory slots.  We need to do that too,
> both to avoid accessing stale copies of the memslots and to avoid
> lockdep warnings.  This therefore adds srcu_read_lock/unlock pairs
> around code that accesses and uses memslots in the Book 3S PR code
> and the Book E (44x and e500) code.
> 
> Signed-off-by: Paul Mackerras <paulus@samba.org>
> ---
> Compile-tested only.
> 
>  arch/powerpc/kvm/44x_tlb.c   |    6 ++++++
>  arch/powerpc/kvm/book3s_pr.c |    6 ++++++
>  arch/powerpc/kvm/e500_tlb.c  |    6 ++++++
>  3 files changed, 18 insertions(+)

On top of the previous comment:

x86 calls srcu_read_lock at the beginning of the KVM_RUN ioctl handler
(__vcpu_run in arch/x86/kvm/x86.c), unlocks srcu on guest entry, locks
on guest exit before any potential use of memslots, and unlocks on
exit to userspace.

This has the advantage of not sprinkling srcu lock/unlock calls all over
(except from other ioctls, of course). Its low maintenance.

Perhaps doing the same on PPC is not a bad idea.