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

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Message-ID: <20250121171756.1e2a2603@p-imbrenda>
Date: Tue, 21 Jan 2025 17:17:56 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Christoph Schlameuss <schlameuss@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Tao Su
 <tao1.su@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v2 3/5] KVM: Add a dedicated API for setting
 KVM-internal memslots

On Tue, 21 Jan 2025 08:05:57 -0800
Sean Christopherson <seanjc@...gle.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@...ux.intel.com>
> > > Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > > Cc: Christian Borntraeger <borntraeger@...ibm.com>
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.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