lkml.org 
[lkml]   [2017]   [Jul]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 3/4] KVM: VMX: simplify and fix vmx_vcpu_pi_load


On 2017/6/6 18:57, Paolo Bonzini wrote:

> The simplify part: do not touch pi_desc.nv, we can set it when the
> VCPU is first created. Likewise, pi_desc.sn is only handled by
> vmx_vcpu_pi_load, do not touch it in __pi_post_block.
>
> The fix part: do not check kvm_arch_has_assigned_device, instead
> check the SN bit to figure out whether vmx_vcpu_pi_put ran before.
> This matches what the previous patch did in pi_post_block.
>
> Cc: Longpeng (Mike) <longpeng2@huawei.com>
> Cc: Huangweidong <weidong.huang@huawei.com>
> Cc: Gonglei <arei.gonglei@huawei.com>
> Cc: wangxin <wangxinxin.wang@huawei.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx.c | 68 ++++++++++++++++++++++++++++--------------------------
> 1 file changed, 35 insertions(+), 33 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 0f4714fe4908..81047f373747 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2184,43 +2184,41 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
> struct pi_desc old, new;
> unsigned int dest;
>
> - if (!kvm_arch_has_assigned_device(vcpu->kvm) ||
> - !irq_remapping_cap(IRQ_POSTING_CAP) ||
> - !kvm_vcpu_apicv_active(vcpu))
> + /*
> + * In case of hot-plug or hot-unplug, we may have to undo
> + * vmx_vcpu_pi_put even if there is no assigned device. And we
> + * always keep PI.NDST up to date for simplicity: it makes the
> + * code easier, and CPU migration is not a fast path.
> + */
> + if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
> + return;


Hi Paolo,

I'm confused with the following scenario:

(suppose the VM has a assigned devices)
step 1. the running vcpu is be preempted
--> vmx_vcpu_pi_put [ SET pi.sn ]
step 2. hot-unplug the assigned devices
step 3. the vcpu is scheduled in
--> vmx_vcpu_pi_load [ CLEAR pi.sn ]
step 4. the running vcpu is be preempted again
--> vmx_vcpu_pi_put [ direct return ]
step 5. the vcpu is migrated to another pcpu
step 6. the vcpu is scheduled in
--> vmx_vcpu_pi_load [ above check fails and
continue to execute the follow parts ]

I think vmx_vcpu_pi_load should return direct in step6, because
vmx_vcpu_pi_put in step4 did nothing.
So maybe the above check has a potential problem.

Please kindly figure out if I misunderstand anything important :)

--
Regards,
Longpeng(Mike)

> +
> + /*
> + * First handle the simple case where no cmpxchg is necessary; just
> + * allow posting non-urgent interrupts.
> + *
> + * If the 'nv' field is POSTED_INTR_WAKEUP_VECTOR, do not change
> + * PI.NDST: pi_post_block will do it for us and the wakeup_handler
> + * expects the VCPU to be on the blocked_vcpu_list that matches
> + * PI.NDST.
> + */
> + if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR ||
> + vcpu->cpu == cpu) {
> + pi_clear_sn(pi_desc);
> return;
> + }
>
> + /* The full case. */
> do {
> old.control = new.control = pi_desc->control;
>
> - /*
> - * If 'nv' field is POSTED_INTR_WAKEUP_VECTOR, there
> - * are two possible cases:
> - * 1. After running 'pre_block', context switch
> - * happened. For this case, 'sn' was set in
> - * vmx_vcpu_put(), so we need to clear it here.
> - * 2. After running 'pre_block', we were blocked,
> - * and woken up by some other guy. For this case,
> - * we don't need to do anything, 'pi_post_block'
> - * will do everything for us. However, we cannot
> - * check whether it is case #1 or case #2 here
> - * (maybe, not needed), so we also clear sn here,
> - * I think it is not a big deal.
> - */
> - if (pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR) {
> - if (vcpu->cpu != cpu) {
> - dest = cpu_physical_id(cpu);
> -
> - if (x2apic_enabled())
> - new.ndst = dest;
> - else
> - new.ndst = (dest << 8) & 0xFF00;
> - }
> + dest = cpu_physical_id(cpu);
>
> - /* set 'NV' to 'notification vector' */
> - new.nv = POSTED_INTR_VECTOR;
> - }
> + if (x2apic_enabled())
> + new.ndst = dest;
> + else
> + new.ndst = (dest << 8) & 0xFF00;
>
> - /* Allow posting non-urgent interrupts */
> new.sn = 0;
> } while (cmpxchg(&pi_desc->control, old.control,
> new.control) != old.control);
> @@ -9259,6 +9257,13 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
>
> vmx->msr_ia32_feature_control_valid_bits = FEATURE_CONTROL_LOCKED;
>
> + /*
> + * Enforce invariant: pi_desc.nv is always either POSTED_INTR_VECTOR
> + * or POSTED_INTR_WAKEUP_VECTOR.
> + */
> + vmx->pi_desc.nv = POSTED_INTR_VECTOR;
> + vmx->pi_desc.sn = 1;
> +
> return &vmx->vcpu;
>
> free_vmcs:
> @@ -11249,9 +11254,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
> else
> new.ndst = (dest << 8) & 0xFF00;
>
> - /* Allow posting non-urgent interrupts */
> - new.sn = 0;
> -
> /* set 'NV' to 'notification vector' */
> new.nv = POSTED_INTR_VECTOR;
> } while (cmpxchg(&pi_desc->control, old.control,


--
Regards,
Longpeng(Mike)

\
 
 \ /
  Last update: 2017-07-28 06:23    [W:0.277 / U:0.192 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site