Re: [PATCH v1 2/3] KVM: VMX: Do not change PID.NDST when loading a blocked vCPU

Joao Martins
2019-11-06T21:55:31Z

On 11/6/19 5:56 PM, Joao Martins wrote:
> When vCPU enters block phase, pi_pre_block() inserts vCPU to a per pCPU
> linked list of all vCPUs that are blocked on this pCPU. Afterwards, it
> changes PID.NV to POSTED_INTR_WAKEUP_VECTOR which its handler
> (wakeup_handler()) is responsible to kick (unblock) any vCPU on that
> linked list that now has pending posted interrupts.
>
> While vCPU is blocked (in kvm_vcpu_block()), it may be preempted which
> will cause vmx_vcpu_pi_put() to set PID.SN. If later the vCPU will be
> scheduled to run on a different pCPU, vmx_vcpu_pi_load() will clear
> PID.SN but will also *overwrite PID.NDST to this different pCPU*.
> Instead of keeping it with original pCPU which vCPU had entered block
> phase on.
>
> This results in an issue because when a posted interrupt is delivered,
> the wakeup_handler() will be executed and fail to find blocked vCPU on
> its per pCPU linked list of all vCPUs that are blocked on this pCPU.
> Which is due to the vCPU being placed on a *different* per pCPU
> linked list than the original pCPU that it had entered block phase.
>
> The regression is introduced by commit c112b5f50232 ("KVM: x86:
> Recompute PID.ON when clearing PID.SN"). Therefore, partially revert
> it and reintroduce the condition in vmx_vcpu_pi_load() responsible for
> avoiding changing PID.NDST when loading a blocked vCPU.
>
> Fixes: c112b5f50232 ("KVM: x86: Recompute PID.ON when clearing PID.SN")
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>

Sorry, I missed a Tb tag:

Tested-by: Nathan Ni <nathan.ni@oracle.com>

I can resend it, if preferred.

> ---
> arch/x86/kvm/vmx/vmx.c | 14 ++++++++++++++
> arch/x86/kvm/vmx/vmx.h | 6 ++++++
> 2 files changed, 20 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 18b0bee662a5..75d903455e1c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1274,6 +1274,18 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
> if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
> return;
>
> + /*
> + * If the 'nv' field is POSTED_INTR_WAKEUP_VECTOR, do not change
> + * PI.NDST: pi_post_block is the one expected to change PID.NDST and the
> + * wakeup handler expects the vCPU to be on the blocked_vcpu_list that
> + * matches PI.NDST. Otherwise, a vcpu may not be able to be woken up
> + * correctly.
> + */
> + if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR || vcpu->cpu == cpu) {
> + pi_clear_sn(pi_desc);
> + goto after_clear_sn;
> + }
> +
> /* The full case. */
> do {
> old.control = new.control = pi_desc->control;
> @@ -1289,6 +1301,8 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
> } while (cmpxchg64(&pi_desc->control, old.control,
> new.control) != old.control);
>
> +after_clear_sn:
> +
> /*
> * Clear SN before reading the bitmap. The VT-d firmware
> * writes the bitmap and reads SN atomically (5.2.3 in the
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index bee16687dc0b..1e32ab54fc2d 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -373,6 +373,12 @@ static inline void pi_clear_on(struct pi_desc *pi_desc)
> (unsigned long *)&pi_desc->control);
> }
>
> +static inline void pi_clear_sn(struct pi_desc *pi_desc)
> +{
> + clear_bit(POSTED_INTR_SN,
> + (unsigned long *)&pi_desc->control);
> +}
> +
> static inline int pi_test_on(struct pi_desc *pi_desc)
> {
> return test_bit(POSTED_INTR_ON,
>