linux-kernel - Re: [PATCH] RISC-V: KVM: Fix kvm_riscv_vcpu_timer

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Message-ID: <CAOnJCUJYQegEa3H+1drGAcy5ptEku9A3gtKWkOm=imC62S4UZw@mail.gmail.com>
Date:   Thu, 20 Oct 2022 23:04:35 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Anup Patel <apatel@...tanamicro.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Anup Patel <anup@...infault.org>, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RISC-V: KVM: Fix kvm_riscv_vcpu_timer_pending() for Sstc

On Wed, Oct 19, 2022 at 4:45 AM Anup Patel <apatel@...tanamicro.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@...tanamicro.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@...osinc.com>
-- 
Regards,
Atish