Re: [PATCH] RISC-V: KVM: Fix kvm_riscv_vcpu_timer_pending() for Sstc

[Anup Patel <anup@brainfault.org>]

On Fri, Oct 21, 2022 at 11:34 AM Atish Patra <atishp@atishpatra.org> wrote:
>
> On Wed, Oct 19, 2022 at 4:45 AM Anup Patel <apatel@ventanamicro.com> wrote:
> >
> > The kvm_riscv_vcpu_timer_pending() checks per-VCPU next_cycles
> > and per-VCPU software injected VS timer interrupt. This function
> > returns incorrect value when Sstc is available because the per-VCPU
> > next_cycles are only updated by kvm_riscv_vcpu_timer_save() called
> > from kvm_arch_vcpu_put(). As a result, when Sstc is available the
> > VCPU does not block properly upon WFI traps.
> >
> > To fix the above issue, we introduce kvm_riscv_vcpu_timer_sync()
> > which will update per-VCPU next_cycles upon every VM exit instead
> > of kvm_riscv_vcpu_timer_save().
> >
> > Fixes: 8f5cb44b1bae ("RISC-V: KVM: Support sstc extension")
> > Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> > ---
> >  arch/riscv/include/asm/kvm_vcpu_timer.h |  1 +
> >  arch/riscv/kvm/vcpu.c                   |  3 +++
> >  arch/riscv/kvm/vcpu_timer.c             | 17 +++++++++++++++--
> >  3 files changed, 19 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/kvm_vcpu_timer.h b/arch/riscv/include/asm/kvm_vcpu_timer.h
> > index 0d8fdb8ec63a..82f7260301da 100644
> > --- a/arch/riscv/include/asm/kvm_vcpu_timer.h
> > +++ b/arch/riscv/include/asm/kvm_vcpu_timer.h
> > @@ -45,6 +45,7 @@ int kvm_riscv_vcpu_timer_deinit(struct kvm_vcpu *vcpu);
> >  int kvm_riscv_vcpu_timer_reset(struct kvm_vcpu *vcpu);
> >  void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu *vcpu);
> >  void kvm_riscv_guest_timer_init(struct kvm *kvm);
> > +void kvm_riscv_vcpu_timer_sync(struct kvm_vcpu *vcpu);
> >  void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu);
> >  bool kvm_riscv_vcpu_timer_pending(struct kvm_vcpu *vcpu);
> >
> > diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> > index a032c4f0d600..71ebbc4821f0 100644
> > --- a/arch/riscv/kvm/vcpu.c
> > +++ b/arch/riscv/kvm/vcpu.c
> > @@ -708,6 +708,9 @@ void kvm_riscv_vcpu_sync_interrupts(struct kvm_vcpu *vcpu)
> >                                 clear_bit(IRQ_VS_SOFT, &v->irqs_pending);
> >                 }
> >         }
> > +
> > +       /* Sync-up timer CSRs */
> > +       kvm_riscv_vcpu_timer_sync(vcpu);
> >  }
> >
> >  int kvm_riscv_vcpu_set_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
> > diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
> > index 185f2386a747..ad34519c8a13 100644
> > --- a/arch/riscv/kvm/vcpu_timer.c
> > +++ b/arch/riscv/kvm/vcpu_timer.c
> > @@ -320,20 +320,33 @@ void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu *vcpu)
> >         kvm_riscv_vcpu_timer_unblocking(vcpu);
> >  }
> >
> > -void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu)
> > +void kvm_riscv_vcpu_timer_sync(struct kvm_vcpu *vcpu)
> >  {
> >         struct kvm_vcpu_timer *t = &vcpu->arch.timer;
> >
> >         if (!t->sstc_enabled)
> >                 return;
> >
> > -       t = &vcpu->arch.timer;
> >  #if defined(CONFIG_32BIT)
> >         t->next_cycles = csr_read(CSR_VSTIMECMP);
> >         t->next_cycles |= (u64)csr_read(CSR_VSTIMECMPH) << 32;
> >  #else
> >         t->next_cycles = csr_read(CSR_VSTIMECMP);
> >  #endif
> > +}
> > +
> > +void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu)
> > +{
> > +       struct kvm_vcpu_timer *t = &vcpu->arch.timer;
> > +
> > +       if (!t->sstc_enabled)
> > +               return;
> > +
> > +       /*
> > +        * The vstimecmp CSRs are saved by kvm_riscv_vcpu_timer_sync()
> > +        * upon every VM exit so no need to save here.
> > +        */
> > +
> >         /* timer should be enabled for the remaining operations */
> >         if (unlikely(!t->init_done))
> >                 return;
> > --
> > 2.34.1
> >
>
> Ahh. That's a tricky one. Thanks for fixing it.
>
> Reviewed-by: Atish Patra <atishp@rivosinc.com>

Thanks, queued this patch for Linux-6.1-rcX

Regards,
Anup