[PATCH 1/1] RISC-V: KVM: Avoid lock inversion in SBI_EXT_HSM_HART_START

[Yong-Xuan Wang]

Hi Anup,

On Tue, Apr 2, 2024 at 11:48 AM Anup Patel <anup at brainfault.org> wrote:
>
> On Tue, Mar 26, 2024 at 3:41 PM Yong-Xuan Wang <yongxuan.wang at sifive.com> wrote:
> >
> > Documentation/virt/kvm/locking.rst advises that kvm->lock should be
> > acquired outside vcpu->mutex and kvm->srcu. However, when KVM/RISC-V
> > handling SBI_EXT_HSM_HART_START, the lock ordering is vcpu->mutex,
> > kvm->srcu then kvm->lock.
> >
> > Although the lockdep checking no longer complains about this after commit
> > f0f44752f5f6 ("rcu: Annotate SRCU's update-side lockdep dependencies"),
> > it's necessary to replace kvm->lock with a new dedicated lock to ensure
> > only one hart can execute the SBI_EXT_HSM_HART_START call for the target
> > hart simultaneously.
> >
> > Signed-off-by: Yong-Xuan Wang <yongxuan.wang at sifive.com>
> > ---
> >  arch/riscv/include/asm/kvm_host.h | 1 +
> >  arch/riscv/kvm/vcpu.c             | 1 +
> >  arch/riscv/kvm/vcpu_sbi_hsm.c     | 5 ++---
> >  3 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
> > index 484d04a92fa6..537099413344 100644
> > --- a/arch/riscv/include/asm/kvm_host.h
> > +++ b/arch/riscv/include/asm/kvm_host.h
> > @@ -254,6 +254,7 @@ struct kvm_vcpu_arch {
> >
> >         /* VCPU power-off state */
> >         bool power_off;
> > +       struct mutex hsm_start_lock;
>
> Instead of a mutex hsm_start_lock, let's introduce spinlock mp_state_lock
> which needs to be taken whenever power_off is accessed. Also, we should
> rename "power_off" to "mp_state" with two possible values.
>
> >
> >         /* Don't run the VCPU (blocked) */
> >         bool pause;
> > diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> > index b5ca9f2e98ac..4d89b5b5afbf 100644
> > --- a/arch/riscv/kvm/vcpu.c
> > +++ b/arch/riscv/kvm/vcpu.c
> > @@ -119,6 +119,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >         spin_lock_init(&vcpu->arch.hfence_lock);
> >
> >         /* Setup reset state of shadow SSTATUS and HSTATUS CSRs */
> > +       mutex_init(&vcpu->arch.hsm_start_lock);
> >         cntx = &vcpu->arch.guest_reset_context;
> >         cntx->sstatus = SR_SPP | SR_SPIE;
> >         cntx->hstatus = 0;
> > diff --git a/arch/riscv/kvm/vcpu_sbi_hsm.c b/arch/riscv/kvm/vcpu_sbi_hsm.c
> > index 7dca0e9381d9..b528f6e880ae 100644
> > --- a/arch/riscv/kvm/vcpu_sbi_hsm.c
> > +++ b/arch/riscv/kvm/vcpu_sbi_hsm.c
> > @@ -71,14 +71,13 @@ static int kvm_sbi_ext_hsm_handler(struct kvm_vcpu *vcpu, struct kvm_run *run,
> >  {
> >         int ret = 0;
> >         struct kvm_cpu_context *cp = &vcpu->arch.guest_context;
> > -       struct kvm *kvm = vcpu->kvm;
> >         unsigned long funcid = cp->a6;
> >
> >         switch (funcid) {
> >         case SBI_EXT_HSM_HART_START:
> > -               mutex_lock(&kvm->lock);
> > +               mutex_lock(&vcpu->arch.hsm_start_lock);
> >                 ret = kvm_sbi_hsm_vcpu_start(vcpu);
> > -               mutex_unlock(&kvm->lock);
> > +               mutex_unlock(&vcpu->arch.hsm_start_lock);
>
> The use of kvm->lock over here was also protecting
> simultaneous updates to VCPU reset context. It's better
> to introduce a separate lock for protecting VCPU reset
> context access.
>
> >                 break;
> >         case SBI_EXT_HSM_HART_STOP:
> >                 ret = kvm_sbi_hsm_vcpu_stop(vcpu);
> > --
> > 2.17.1
> >
>
> I think this patch can be broken down into two patches:
> 1) Patch replacing VCPU "power_off" with "enum mp_state"
>     and introducing "mp_state_lock"
> 2) Patch introducing VCPU reset context lock
>
> Regards,
> Anup

Got it! I will update these in a new patchset. Thank you!

Regards,
Yong-Xuan