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* [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision
@ 2019-08-12 18:20 Haiyang Zhang
  2019-08-13 10:14 ` Lorenzo Pieralisi
  0 siblings, 1 reply; 5+ messages in thread
From: Haiyang Zhang @ 2019-08-12 18:20 UTC (permalink / raw)
  To: sashal, bhelgaas, lorenzo.pieralisi, linux-hyperv, linux-pci
  Cc: Haiyang Zhang, KY Srinivasan, Stephen Hemminger, olaf, vkuznets,
	linux-kernel

Currently in Azure cloud, for passthrough devices including GPU, the host
sets the device instance ID's bytes 8 - 15 to a value derived from the host
HWID, which is the same on all devices in a VM. So, the device instance
ID's bytes 8 and 9 provided by the host are no longer unique. This can
cause device passthrough to VMs to fail because the bytes 8 and 9 are used
as PCI domain number. Collision of domain numbers will cause the second
device with the same domain number fail to load.

As recommended by Azure host team, the bytes 4, 5 have more uniqueness
(info entropy) than bytes 8, 9. So now we use bytes 4, 5 as the PCI domain
numbers. On older hosts, bytes 4, 5 can also be used -- no backward
compatibility issues here. The chance of collision is greatly reduced. In
the rare cases of collision, we will detect and find another number that is
not in use.

Suggested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Acked-by: Sasha Levin <sashal@kernel.org>
---
 drivers/pci/controller/pci-hyperv.c | 92 +++++++++++++++++++++++++++++++------
 1 file changed, 79 insertions(+), 13 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index 40b6254..4f3d97e 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -2510,6 +2510,48 @@ static void put_hvpcibus(struct hv_pcibus_device *hbus)
 		complete(&hbus->remove_event);
 }
 
+#define HVPCI_DOM_MAP_SIZE (64 * 1024)
+static DECLARE_BITMAP(hvpci_dom_map, HVPCI_DOM_MAP_SIZE);
+
+/*
+ * PCI domain number 0 is used by emulated devices on Gen1 VMs, so define 0
+ * as invalid for passthrough PCI devices of this driver.
+ */
+#define HVPCI_DOM_INVALID 0
+
+/**
+ * hv_get_dom_num() - Get a valid PCI domain number
+ * Check if the PCI domain number is in use, and return another number if
+ * it is in use.
+ *
+ * @dom: Requested domain number
+ *
+ * return: domain number on success, HVPCI_DOM_INVALID on failure
+ */
+static u16 hv_get_dom_num(u16 dom)
+{
+	unsigned int i;
+
+	if (test_and_set_bit(dom, hvpci_dom_map) == 0)
+		return dom;
+
+	for_each_clear_bit(i, hvpci_dom_map, HVPCI_DOM_MAP_SIZE) {
+		if (test_and_set_bit(i, hvpci_dom_map) == 0)
+			return i;
+	}
+
+	return HVPCI_DOM_INVALID;
+}
+
+/**
+ * hv_put_dom_num() - Mark the PCI domain number as free
+ * @dom: Domain number to be freed
+ */
+static void hv_put_dom_num(u16 dom)
+{
+	clear_bit(dom, hvpci_dom_map);
+}
+
 /**
  * hv_pci_probe() - New VMBus channel probe, for a root PCI bus
  * @hdev:	VMBus's tracking struct for this root PCI bus
@@ -2521,6 +2563,7 @@ static int hv_pci_probe(struct hv_device *hdev,
 			const struct hv_vmbus_device_id *dev_id)
 {
 	struct hv_pcibus_device *hbus;
+	u16 dom_req, dom;
 	int ret;
 
 	/*
@@ -2535,19 +2578,34 @@ static int hv_pci_probe(struct hv_device *hdev,
 	hbus->state = hv_pcibus_init;
 
 	/*
-	 * The PCI bus "domain" is what is called "segment" in ACPI and
-	 * other specs.  Pull it from the instance ID, to get something
-	 * unique.  Bytes 8 and 9 are what is used in Windows guests, so
-	 * do the same thing for consistency.  Note that, since this code
-	 * only runs in a Hyper-V VM, Hyper-V can (and does) guarantee
-	 * that (1) the only domain in use for something that looks like
-	 * a physical PCI bus (which is actually emulated by the
-	 * hypervisor) is domain 0 and (2) there will be no overlap
-	 * between domains derived from these instance IDs in the same
-	 * VM.
+	 * The PCI bus "domain" is what is called "segment" in ACPI and other
+	 * specs. Pull it from the instance ID, to get something usually
+	 * unique. In rare cases of collision, we will find out another number
+	 * not in use.
+	 *
+	 * Note that, since this code only runs in a Hyper-V VM, Hyper-V
+	 * together with this guest driver can guarantee that (1) The only
+	 * domain used by Gen1 VMs for something that looks like a physical
+	 * PCI bus (which is actually emulated by the hypervisor) is domain 0.
+	 * (2) There will be no overlap between domains (after fixing possible
+	 * collisions) in the same VM.
 	 */
-	hbus->sysdata.domain = hdev->dev_instance.b[9] |
-			       hdev->dev_instance.b[8] << 8;
+	dom_req = hdev->dev_instance.b[5] << 8 | hdev->dev_instance.b[4];
+	dom = hv_get_dom_num(dom_req);
+
+	if (dom == HVPCI_DOM_INVALID) {
+		dev_err(&hdev->device,
+			"Unable to use dom# 0x%hx or other numbers", dom_req);
+		ret = -EINVAL;
+		goto free_bus;
+	}
+
+	if (dom != dom_req)
+		dev_info(&hdev->device,
+			 "PCI dom# 0x%hx has collision, using 0x%hx",
+			 dom_req, dom);
+
+	hbus->sysdata.domain = dom;
 
 	hbus->hdev = hdev;
 	refcount_set(&hbus->remove_lock, 1);
@@ -2562,7 +2620,7 @@ static int hv_pci_probe(struct hv_device *hdev,
 					   hbus->sysdata.domain);
 	if (!hbus->wq) {
 		ret = -ENOMEM;
-		goto free_bus;
+		goto free_dom;
 	}
 
 	ret = vmbus_open(hdev->channel, pci_ring_size, pci_ring_size, NULL, 0,
@@ -2639,6 +2697,8 @@ static int hv_pci_probe(struct hv_device *hdev,
 	vmbus_close(hdev->channel);
 destroy_wq:
 	destroy_workqueue(hbus->wq);
+free_dom:
+	hv_put_dom_num(hbus->sysdata.domain);
 free_bus:
 	free_page((unsigned long)hbus);
 	return ret;
@@ -2720,6 +2780,9 @@ static int hv_pci_remove(struct hv_device *hdev)
 	put_hvpcibus(hbus);
 	wait_for_completion(&hbus->remove_event);
 	destroy_workqueue(hbus->wq);
+
+	hv_put_dom_num(hbus->sysdata.domain);
+
 	free_page((unsigned long)hbus);
 	return 0;
 }
@@ -2747,6 +2810,9 @@ static void __exit exit_hv_pci_drv(void)
 
 static int __init init_hv_pci_drv(void)
 {
+	/* Set the invalid domain number's bit, so it will not be used */
+	set_bit(HVPCI_DOM_INVALID, hvpci_dom_map);
+
 	return vmbus_driver_register(&hv_pci_drv);
 }
 
-- 
1.8.3.1


^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision
  2019-08-12 18:20 [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision Haiyang Zhang
@ 2019-08-13 10:14 ` Lorenzo Pieralisi
  2019-08-13 12:55   ` Haiyang Zhang
  0 siblings, 1 reply; 5+ messages in thread
From: Lorenzo Pieralisi @ 2019-08-13 10:14 UTC (permalink / raw)
  To: Haiyang Zhang
  Cc: sashal, bhelgaas, linux-hyperv, linux-pci, KY Srinivasan,
	Stephen Hemminger, olaf, vkuznets, linux-kernel

On Mon, Aug 12, 2019 at 06:20:53PM +0000, Haiyang Zhang wrote:
> Currently in Azure cloud, for passthrough devices including GPU, the host
> sets the device instance ID's bytes 8 - 15 to a value derived from the host
> HWID, which is the same on all devices in a VM. So, the device instance
> ID's bytes 8 and 9 provided by the host are no longer unique. This can
> cause device passthrough to VMs to fail because the bytes 8 and 9 are used
> as PCI domain number. Collision of domain numbers will cause the second
> device with the same domain number fail to load.
> 
> As recommended by Azure host team, the bytes 4, 5 have more uniqueness
> (info entropy) than bytes 8, 9. So now we use bytes 4, 5 as the PCI domain
> numbers. On older hosts, bytes 4, 5 can also be used -- no backward
> compatibility issues here. The chance of collision is greatly reduced. In
> the rare cases of collision, we will detect and find another number that is
> not in use.

I have not explained what I meant correctly. This patch fixes an
issue and the "find another number" fallback can be also applied
to the current kernel without changing the bytes you use for
domain numbers.

This patch would leave old kernels susceptible to breakage.

Again, I have no Azure knowledge but it seems better to me to
add a fallback "find another number" allocation on top of mainline
and send it to stable kernels. Then we can add another patch to
change the bytes you use to reduce the number of collision.

Please let me know what you think, thanks.

Lorenzo

> Suggested-by: Michael Kelley <mikelley@microsoft.com>
> Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
> Acked-by: Sasha Levin <sashal@kernel.org>
> ---
>  drivers/pci/controller/pci-hyperv.c | 92 +++++++++++++++++++++++++++++++------
>  1 file changed, 79 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 40b6254..4f3d97e 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -2510,6 +2510,48 @@ static void put_hvpcibus(struct hv_pcibus_device *hbus)
>  		complete(&hbus->remove_event);
>  }
>  
> +#define HVPCI_DOM_MAP_SIZE (64 * 1024)
> +static DECLARE_BITMAP(hvpci_dom_map, HVPCI_DOM_MAP_SIZE);
> +
> +/*
> + * PCI domain number 0 is used by emulated devices on Gen1 VMs, so define 0
> + * as invalid for passthrough PCI devices of this driver.
> + */
> +#define HVPCI_DOM_INVALID 0
> +
> +/**
> + * hv_get_dom_num() - Get a valid PCI domain number
> + * Check if the PCI domain number is in use, and return another number if
> + * it is in use.
> + *
> + * @dom: Requested domain number
> + *
> + * return: domain number on success, HVPCI_DOM_INVALID on failure
> + */
> +static u16 hv_get_dom_num(u16 dom)
> +{
> +	unsigned int i;
> +
> +	if (test_and_set_bit(dom, hvpci_dom_map) == 0)
> +		return dom;
> +
> +	for_each_clear_bit(i, hvpci_dom_map, HVPCI_DOM_MAP_SIZE) {
> +		if (test_and_set_bit(i, hvpci_dom_map) == 0)
> +			return i;
> +	}
> +
> +	return HVPCI_DOM_INVALID;
> +}
> +
> +/**
> + * hv_put_dom_num() - Mark the PCI domain number as free
> + * @dom: Domain number to be freed
> + */
> +static void hv_put_dom_num(u16 dom)
> +{
> +	clear_bit(dom, hvpci_dom_map);
> +}
> +
>  /**
>   * hv_pci_probe() - New VMBus channel probe, for a root PCI bus
>   * @hdev:	VMBus's tracking struct for this root PCI bus
> @@ -2521,6 +2563,7 @@ static int hv_pci_probe(struct hv_device *hdev,
>  			const struct hv_vmbus_device_id *dev_id)
>  {
>  	struct hv_pcibus_device *hbus;
> +	u16 dom_req, dom;
>  	int ret;
>  
>  	/*
> @@ -2535,19 +2578,34 @@ static int hv_pci_probe(struct hv_device *hdev,
>  	hbus->state = hv_pcibus_init;
>  
>  	/*
> -	 * The PCI bus "domain" is what is called "segment" in ACPI and
> -	 * other specs.  Pull it from the instance ID, to get something
> -	 * unique.  Bytes 8 and 9 are what is used in Windows guests, so
> -	 * do the same thing for consistency.  Note that, since this code
> -	 * only runs in a Hyper-V VM, Hyper-V can (and does) guarantee
> -	 * that (1) the only domain in use for something that looks like
> -	 * a physical PCI bus (which is actually emulated by the
> -	 * hypervisor) is domain 0 and (2) there will be no overlap
> -	 * between domains derived from these instance IDs in the same
> -	 * VM.
> +	 * The PCI bus "domain" is what is called "segment" in ACPI and other
> +	 * specs. Pull it from the instance ID, to get something usually
> +	 * unique. In rare cases of collision, we will find out another number
> +	 * not in use.
> +	 *
> +	 * Note that, since this code only runs in a Hyper-V VM, Hyper-V
> +	 * together with this guest driver can guarantee that (1) The only
> +	 * domain used by Gen1 VMs for something that looks like a physical
> +	 * PCI bus (which is actually emulated by the hypervisor) is domain 0.
> +	 * (2) There will be no overlap between domains (after fixing possible
> +	 * collisions) in the same VM.
>  	 */
> -	hbus->sysdata.domain = hdev->dev_instance.b[9] |
> -			       hdev->dev_instance.b[8] << 8;
> +	dom_req = hdev->dev_instance.b[5] << 8 | hdev->dev_instance.b[4];
> +	dom = hv_get_dom_num(dom_req);
> +
> +	if (dom == HVPCI_DOM_INVALID) {
> +		dev_err(&hdev->device,
> +			"Unable to use dom# 0x%hx or other numbers", dom_req);
> +		ret = -EINVAL;
> +		goto free_bus;
> +	}
> +
> +	if (dom != dom_req)
> +		dev_info(&hdev->device,
> +			 "PCI dom# 0x%hx has collision, using 0x%hx",
> +			 dom_req, dom);
> +
> +	hbus->sysdata.domain = dom;
>  
>  	hbus->hdev = hdev;
>  	refcount_set(&hbus->remove_lock, 1);
> @@ -2562,7 +2620,7 @@ static int hv_pci_probe(struct hv_device *hdev,
>  					   hbus->sysdata.domain);
>  	if (!hbus->wq) {
>  		ret = -ENOMEM;
> -		goto free_bus;
> +		goto free_dom;
>  	}
>  
>  	ret = vmbus_open(hdev->channel, pci_ring_size, pci_ring_size, NULL, 0,
> @@ -2639,6 +2697,8 @@ static int hv_pci_probe(struct hv_device *hdev,
>  	vmbus_close(hdev->channel);
>  destroy_wq:
>  	destroy_workqueue(hbus->wq);
> +free_dom:
> +	hv_put_dom_num(hbus->sysdata.domain);
>  free_bus:
>  	free_page((unsigned long)hbus);
>  	return ret;
> @@ -2720,6 +2780,9 @@ static int hv_pci_remove(struct hv_device *hdev)
>  	put_hvpcibus(hbus);
>  	wait_for_completion(&hbus->remove_event);
>  	destroy_workqueue(hbus->wq);
> +
> +	hv_put_dom_num(hbus->sysdata.domain);
> +
>  	free_page((unsigned long)hbus);
>  	return 0;
>  }
> @@ -2747,6 +2810,9 @@ static void __exit exit_hv_pci_drv(void)
>  
>  static int __init init_hv_pci_drv(void)
>  {
> +	/* Set the invalid domain number's bit, so it will not be used */
> +	set_bit(HVPCI_DOM_INVALID, hvpci_dom_map);
> +
>  	return vmbus_driver_register(&hv_pci_drv);
>  }
>  
> -- 
> 1.8.3.1
> 

^ permalink raw reply	[flat|nested] 5+ messages in thread

* RE: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision
  2019-08-13 10:14 ` Lorenzo Pieralisi
@ 2019-08-13 12:55   ` Haiyang Zhang
  2019-08-13 14:25     ` Lorenzo Pieralisi
  0 siblings, 1 reply; 5+ messages in thread
From: Haiyang Zhang @ 2019-08-13 12:55 UTC (permalink / raw)
  To: Lorenzo Pieralisi
  Cc: sashal, bhelgaas, linux-hyperv, linux-pci, KY Srinivasan,
	Stephen Hemminger, olaf, vkuznets, linux-kernel



> -----Original Message-----
> From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Sent: Tuesday, August 13, 2019 6:14 AM
> To: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: sashal@kernel.org; bhelgaas@google.com; linux-
> hyperv@vger.kernel.org; linux-pci@vger.kernel.org; KY Srinivasan
> <kys@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> olaf@aepfle.de; vkuznets <vkuznets@redhat.com>; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number
> collision
> 
> On Mon, Aug 12, 2019 at 06:20:53PM +0000, Haiyang Zhang wrote:
> > Currently in Azure cloud, for passthrough devices including GPU, the host
> > sets the device instance ID's bytes 8 - 15 to a value derived from the host
> > HWID, which is the same on all devices in a VM. So, the device instance
> > ID's bytes 8 and 9 provided by the host are no longer unique. This can
> > cause device passthrough to VMs to fail because the bytes 8 and 9 are used
> > as PCI domain number. Collision of domain numbers will cause the second
> > device with the same domain number fail to load.
> >
> > As recommended by Azure host team, the bytes 4, 5 have more uniqueness
> > (info entropy) than bytes 8, 9. So now we use bytes 4, 5 as the PCI domain
> > numbers. On older hosts, bytes 4, 5 can also be used -- no backward
> > compatibility issues here. The chance of collision is greatly reduced. In
> > the rare cases of collision, we will detect and find another number that is
> > not in use.
> 
> I have not explained what I meant correctly. This patch fixes an
> issue and the "find another number" fallback can be also applied
> to the current kernel without changing the bytes you use for
> domain numbers.
> 
> This patch would leave old kernels susceptible to breakage.
> 
> Again, I have no Azure knowledge but it seems better to me to
> add a fallback "find another number" allocation on top of mainline
> and send it to stable kernels. Then we can add another patch to
> change the bytes you use to reduce the number of collision.
> 
> Please let me know what you think, thanks.

Thanks for your clarification.
Actually, I hope the stable kernel will be patched to use bytes 4,5 too,
because host provided numbers are persistent across reboots, we like
to use them if possible.

I think we can either --
1) Apply this patch for mainline and stable kernels as well.
2) Or, break this patch into two patches, and apply both of them for 
Mainline and stable kernels.

Which way do you prefer?

Thanks,
- Haiyang


^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision
  2019-08-13 12:55   ` Haiyang Zhang
@ 2019-08-13 14:25     ` Lorenzo Pieralisi
  2019-08-13 14:39       ` Haiyang Zhang
  0 siblings, 1 reply; 5+ messages in thread
From: Lorenzo Pieralisi @ 2019-08-13 14:25 UTC (permalink / raw)
  To: Haiyang Zhang
  Cc: sashal, bhelgaas, linux-hyperv, linux-pci, KY Srinivasan,
	Stephen Hemminger, olaf, vkuznets, linux-kernel

On Tue, Aug 13, 2019 at 12:55:59PM +0000, Haiyang Zhang wrote:
> 
> 
> > -----Original Message-----
> > From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > Sent: Tuesday, August 13, 2019 6:14 AM
> > To: Haiyang Zhang <haiyangz@microsoft.com>
> > Cc: sashal@kernel.org; bhelgaas@google.com; linux-
> > hyperv@vger.kernel.org; linux-pci@vger.kernel.org; KY Srinivasan
> > <kys@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> > olaf@aepfle.de; vkuznets <vkuznets@redhat.com>; linux-
> > kernel@vger.kernel.org
> > Subject: Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number
> > collision
> > 
> > On Mon, Aug 12, 2019 at 06:20:53PM +0000, Haiyang Zhang wrote:
> > > Currently in Azure cloud, for passthrough devices including GPU, the host
> > > sets the device instance ID's bytes 8 - 15 to a value derived from the host
> > > HWID, which is the same on all devices in a VM. So, the device instance
> > > ID's bytes 8 and 9 provided by the host are no longer unique. This can
> > > cause device passthrough to VMs to fail because the bytes 8 and 9 are used
> > > as PCI domain number. Collision of domain numbers will cause the second
> > > device with the same domain number fail to load.
> > >
> > > As recommended by Azure host team, the bytes 4, 5 have more uniqueness
> > > (info entropy) than bytes 8, 9. So now we use bytes 4, 5 as the PCI domain
> > > numbers. On older hosts, bytes 4, 5 can also be used -- no backward
> > > compatibility issues here. The chance of collision is greatly reduced. In
> > > the rare cases of collision, we will detect and find another number that is
> > > not in use.
> > 
> > I have not explained what I meant correctly. This patch fixes an
> > issue and the "find another number" fallback can be also applied
> > to the current kernel without changing the bytes you use for
> > domain numbers.
> > 
> > This patch would leave old kernels susceptible to breakage.
> > 
> > Again, I have no Azure knowledge but it seems better to me to
> > add a fallback "find another number" allocation on top of mainline
> > and send it to stable kernels. Then we can add another patch to
> > change the bytes you use to reduce the number of collision.
> > 
> > Please let me know what you think, thanks.
> 
> Thanks for your clarification.
> Actually, I hope the stable kernel will be patched to use bytes 4,5 too,
> because host provided numbers are persistent across reboots, we like
> to use them if possible.
> 
> I think we can either --
> 1) Apply this patch for mainline and stable kernels as well.
> 2) Or, break this patch into two patches, and apply both of them for 
> Mainline and stable kernels.

(2) since one patch is a fix and the other one an (optional - however
important it is) change.

This way if the optional change needs reverting we still have a working
kernel.

In the end it is up to you - I am just expressing what I think is the
most sensible way forward.

Lorenzo

^ permalink raw reply	[flat|nested] 5+ messages in thread

* RE: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision
  2019-08-13 14:25     ` Lorenzo Pieralisi
@ 2019-08-13 14:39       ` Haiyang Zhang
  0 siblings, 0 replies; 5+ messages in thread
From: Haiyang Zhang @ 2019-08-13 14:39 UTC (permalink / raw)
  To: Lorenzo Pieralisi
  Cc: sashal, bhelgaas, linux-hyperv, linux-pci, KY Srinivasan,
	Stephen Hemminger, olaf, vkuznets, linux-kernel



> -----Original Message-----
> From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Sent: Tuesday, August 13, 2019 10:26 AM
> To: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: sashal@kernel.org; bhelgaas@google.com; linux-
> hyperv@vger.kernel.org; linux-pci@vger.kernel.org; KY Srinivasan
> <kys@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> olaf@aepfle.de; vkuznets <vkuznets@redhat.com>; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number
> collision
> 
> On Tue, Aug 13, 2019 at 12:55:59PM +0000, Haiyang Zhang wrote:
> >
> >
> > > -----Original Message-----
> > > From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > > Sent: Tuesday, August 13, 2019 6:14 AM
> > > To: Haiyang Zhang <haiyangz@microsoft.com>
> > > Cc: sashal@kernel.org; bhelgaas@google.com; linux-
> > > hyperv@vger.kernel.org; linux-pci@vger.kernel.org; KY Srinivasan
> > > <kys@microsoft.com>; Stephen Hemminger
> <sthemmin@microsoft.com>;
> > > olaf@aepfle.de; vkuznets <vkuznets@redhat.com>; linux-
> > > kernel@vger.kernel.org
> > > Subject: Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain
> number
> > > collision
> > >
> > > On Mon, Aug 12, 2019 at 06:20:53PM +0000, Haiyang Zhang wrote:
> > > > Currently in Azure cloud, for passthrough devices including GPU, the
> host
> > > > sets the device instance ID's bytes 8 - 15 to a value derived from the
> host
> > > > HWID, which is the same on all devices in a VM. So, the device instance
> > > > ID's bytes 8 and 9 provided by the host are no longer unique. This can
> > > > cause device passthrough to VMs to fail because the bytes 8 and 9 are
> used
> > > > as PCI domain number. Collision of domain numbers will cause the
> second
> > > > device with the same domain number fail to load.
> > > >
> > > > As recommended by Azure host team, the bytes 4, 5 have more
> uniqueness
> > > > (info entropy) than bytes 8, 9. So now we use bytes 4, 5 as the PCI
> domain
> > > > numbers. On older hosts, bytes 4, 5 can also be used -- no backward
> > > > compatibility issues here. The chance of collision is greatly reduced. In
> > > > the rare cases of collision, we will detect and find another number that
> is
> > > > not in use.
> > >
> > > I have not explained what I meant correctly. This patch fixes an
> > > issue and the "find another number" fallback can be also applied
> > > to the current kernel without changing the bytes you use for
> > > domain numbers.
> > >
> > > This patch would leave old kernels susceptible to breakage.
> > >
> > > Again, I have no Azure knowledge but it seems better to me to
> > > add a fallback "find another number" allocation on top of mainline
> > > and send it to stable kernels. Then we can add another patch to
> > > change the bytes you use to reduce the number of collision.
> > >
> > > Please let me know what you think, thanks.
> >
> > Thanks for your clarification.
> > Actually, I hope the stable kernel will be patched to use bytes 4,5 too,
> > because host provided numbers are persistent across reboots, we like
> > to use them if possible.
> >
> > I think we can either --
> > 1) Apply this patch for mainline and stable kernels as well.
> > 2) Or, break this patch into two patches, and apply both of them for
> > Mainline and stable kernels.
> 
> (2) since one patch is a fix and the other one an (optional - however
> important it is) change.
> 
> This way if the optional change needs reverting we still have a working
> kernel.
> 
> In the end it is up to you - I am just expressing what I think is the
> most sensible way forward.

Sure, I agree with you, and will break the patch into two, and resubmit.

Thanks,
- Haiyang

^ permalink raw reply	[flat|nested] 5+ messages in thread

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Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-08-12 18:20 [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision Haiyang Zhang
2019-08-13 10:14 ` Lorenzo Pieralisi
2019-08-13 12:55   ` Haiyang Zhang
2019-08-13 14:25     ` Lorenzo Pieralisi
2019-08-13 14:39       ` Haiyang Zhang

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