RE: [PATCH v3] PCI: hv: Fix a timing issue which causes kdump to fail occasionally

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> -----Original Message-----
> From: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> > Kdump could fail sometime on Hyper-V guest over Accelerated Network
> > interface. This is because the retry in hv_pci_enter_d0() relies on an
> > asynchronous host event arriving before the guest calls
> > hv_send_resources_allocated(). Fix the problem by moving retry to
> > hv_pci_probe(), removing this dependency and making the calling
> > sequence synchronous.
> 
> You have to explain why this code move fixes the problem

How about following commit message:

Kdump could fail sometime on Hyper-V guest over Accelerated Network
nterface. This is because the retry in hv_pci_enter_d0() relies on an
asynchronous host event arriving before the guest calls
hv_send_resources_allocated(). Fix the problem by moving retry to
hv_pci_probe() and start the retry from hv_pci_query_relations() call. 
This causes the host to generate an synchronous event, hence removing 
this unreliable dependency.
 
> and you also have to
> add a comment to the code so that anyone who has to fix it in the future can
> understand why the code is where you are moving it to and why that's a
> solution.

It already has the comment in the code as:
+	 * The retry should start from hv_pci_query_relations() call.

It would be a bug if the retry does not start from this according to the host
Behaviour. So I think no further explanation would be needed.

> 
> > Fixes: c81992e7f4aa ("PCI: hv: Retry PCI bus D0 entry on invalid
> > device state")
> > Signed-off-by: Wei Hu <weh@xxxxxxxxxxxxx>
> 
> Please carry tags and send patches -in-reply-to the previous version to allow
> threading.
> 

Do you mean to add review-by tags? If not would you please elaborate what
'carry tags' means? Sorry I am not familiar to this term.

Thanks,
Wei


> Thanks,
> Lorenzo
> 
> > ---
> >     v2: Adding Fixes tag according to Michael Kelley's review comment.
> >     v3: Fix couple typos and reword commit message to make it clearer.
> >     Thanks the comments from Bjorn Helgaas.
> >
> >  drivers/pci/controller/pci-hyperv.c | 66
> > ++++++++++++++---------------
> >  1 file changed, 32 insertions(+), 34 deletions(-)
> >
> > diff --git a/drivers/pci/controller/pci-hyperv.c
> > b/drivers/pci/controller/pci-hyperv.c
> > index bf40ff09c99d..738ee30f3334 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -2759,10 +2759,8 @@ static int hv_pci_enter_d0(struct hv_device
> *hdev)
> >  	struct pci_bus_d0_entry *d0_entry;
> >  	struct hv_pci_compl comp_pkt;
> >  	struct pci_packet *pkt;
> > -	bool retry = true;
> >  	int ret;
> >
> > -enter_d0_retry:
> >  	/*
> >  	 * Tell the host that the bus is ready to use, and moved into the
> >  	 * powered-on state.  This includes telling the host which region @@
> > -2789,38 +2787,6 @@ static int hv_pci_enter_d0(struct hv_device *hdev)
> >  	if (ret)
> >  		goto exit;
> >
> > -	/*
> > -	 * In certain case (Kdump) the pci device of interest was
> > -	 * not cleanly shut down and resource is still held on host
> > -	 * side, the host could return invalid device status.
> > -	 * We need to explicitly request host to release the resource
> > -	 * and try to enter D0 again.
> > -	 */
> > -	if (comp_pkt.completion_status < 0 && retry) {
> > -		retry = false;
> > -
> > -		dev_err(&hdev->device, "Retrying D0 Entry\n");
> > -
> > -		/*
> > -		 * Hv_pci_bus_exit() calls hv_send_resource_released()
> > -		 * to free up resources of its child devices.
> > -		 * In the kdump kernel we need to set the
> > -		 * wslot_res_allocated to 255 so it scans all child
> > -		 * devices to release resources allocated in the
> > -		 * normal kernel before panic happened.
> > -		 */
> > -		hbus->wslot_res_allocated = 255;
> > -
> > -		ret = hv_pci_bus_exit(hdev, true);
> > -
> > -		if (ret == 0) {
> > -			kfree(pkt);
> > -			goto enter_d0_retry;
> > -		}
> > -		dev_err(&hdev->device,
> > -			"Retrying D0 failed with ret %d\n", ret);
> > -	}
> > -
> >  	if (comp_pkt.completion_status < 0) {
> >  		dev_err(&hdev->device,
> >  			"PCI Pass-through VSP failed D0 Entry with
> status %x\n", @@
> > -3058,6 +3024,7 @@ static int hv_pci_probe(struct hv_device *hdev,
> >  	struct hv_pcibus_device *hbus;
> >  	u16 dom_req, dom;
> >  	char *name;
> > +	bool enter_d0_retry = true;
> >  	int ret;
> >
> >  	/*
> > @@ -3178,11 +3145,42 @@ static int hv_pci_probe(struct hv_device *hdev,
> >  	if (ret)
> >  		goto free_fwnode;
> >
> > +retry:
> >  	ret = hv_pci_query_relations(hdev);
> >  	if (ret)
> >  		goto free_irq_domain;
> >
> >  	ret = hv_pci_enter_d0(hdev);
> > +	/*
> > +	 * In certain case (Kdump) the pci device of interest was
> > +	 * not cleanly shut down and resource is still held on host
> > +	 * side, the host could return invalid device status.
> > +	 * We need to explicitly request host to release the resource
> > +	 * and try to enter D0 again.
> > +	 * The retry should start from hv_pci_query_relations() call.
> > +	 */
> > +	if (ret == -EPROTO && enter_d0_retry) {
> > +		enter_d0_retry = false;
> > +
> > +		dev_err(&hdev->device, "Retrying D0 Entry\n");
> > +
> > +		/*
> > +		 * Hv_pci_bus_exit() calls hv_send_resources_released()
> > +		 * to free up resources of its child devices.
> > +		 * In the kdump kernel we need to set the
> > +		 * wslot_res_allocated to 255 so it scans all child
> > +		 * devices to release resources allocated in the
> > +		 * normal kernel before panic happened.
> > +		 */
> > +		hbus->wslot_res_allocated = 255;
> > +		ret = hv_pci_bus_exit(hdev, true);
> > +
> > +		if (ret == 0)
> > +			goto retry;
> > +
> > +		dev_err(&hdev->device,
> > +			"Retrying D0 failed with ret %d\n", ret);
> > +	}
> >  	if (ret)
> >  		goto free_irq_domain;
> >
> > --
> > 2.20.1
> >




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