Re: [PATCH v2 3/5] KVM: Add a dedicated API for setting KVM-internal memslots

["Christoph Schlameuss" <schlameuss@linux.ibm.com>]

On Tue Jan 21, 2025 at 5:17 PM CET, Claudio Imbrenda wrote:
> On Tue, 21 Jan 2025 08:05:57 -0800
> Sean Christopherson <seanjc@google.com> wrote:
>
> > On Mon, Jan 20, 2025, Christoph Schlameuss wrote:
> > > On Sat Jan 11, 2025 at 1:20 AM CET, Sean Christopherson wrote:  
> > > > Add a dedicated API for setting internal memslots, and have it explicitly
> > > > disallow setting userspace memslots.  Setting a userspace memslots without
> > > > a direct command from userspace would result in all manner of issues.
> > > >
> > > > No functional change intended.
> > > >
> > > > Cc: Tao Su <tao1.su@linux.intel.com>
> > > > Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > > > Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> > > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > > ---
> > > >  arch/x86/kvm/x86.c       |  2 +-
> > > >  include/linux/kvm_host.h |  4 ++--
> > > >  virt/kvm/kvm_main.c      | 15 ++++++++++++---
> > > >  3 files changed, 15 insertions(+), 6 deletions(-)  
> > > 
> > > [...]
> > >   
> > > > +int kvm_set_internal_memslot(struct kvm *kvm,
> > > > +			     const struct kvm_userspace_memory_region2 *mem)
> > > > +{
> > > > +	if (WARN_ON_ONCE(mem->slot < KVM_USER_MEM_SLOTS))
> > > > +		return -EINVAL;
> > > > +  
> > > 
> > > Looking at Claudios changes I found that this is missing to acquire the
> > > slots_lock here.
> > > 
> > > guard(mutex)(&kvm->slots_lock);  
> > 
> > It's not missing.  As of this patch, x86 is the only user of KVM-internal memslots,
> > and x86 acquires slots_lock outside of kvm_set_internal_memslot() because x86 can
> > have multiple address spaces (regular vs SMM) and KVM's internal memslots need to
> > be created for both, i.e. it's desirable to holds slots_lock in the caller.
> > 
> > If it's annoying for s390 to acquire slots_lock, we could add a wrapper, i.e. turn
> > this into __kvm_set_internal_memslot() and then re-add kvm_set_internal_memslot()
> > as a version that acquires and releases slots_lock.
>
> I think it's fine as it is, just document that the lock needs to be
> held
>
> I'll add the necessary locking in the s390 code

I see. Thank you for the elaboration, Sean!