[PATCH v5 0/6] drivers:hv: Ensure that bridge windows don't overlap

[PATCH v5 0/6] drivers:hv: Ensure that bridge windows don't overlap

[Jake Oshins]

This version incorporates feedback from Bjorn Helgaas, folding two
patches together.

Hyper-V VMs expose paravirtual drivers through a mechanism called
VMBus, which is managed by hv_vmbus.ko.  For each parvirtual service
instance, this driver exposes a new child device.  Some of these child
devices need memory address space, into which Hyper-V will map things
like the virtual video frame buffer.  This memory-mapped address space
is chosen by the guest OS, not the hypervisor.

This is difficult to map onto the Linux pnp layer, as the code in the
pnp layer to choose MMIO space keys off of bus type and it doesn't know
anything about VMBus.  The maintainers of the pnp layer have asked that
we not offer patches to it that make it understand VMBus, but that we
rather find ways of using the code in its current state.  So hv_vmbus.ko
exports a function, vmbus_allocate_mmio() for choosing the address space
for any child driver that needs this facility.

The recently introduced PCI front-end driver for Hyper-V VMs
(pci-hyperv.ko) uses vmbus_allocate_mmio() for choosing both the region
of memory space into which real PCI Express devices are mapped.  The
regions allocated are made to look like root PCI bus bridge windows
to the PCI driver, reusing all the code in the PCI driver for the rest
of PCI device management.

The problem is that these bridge windows are marked in such a way that
devices can still allocate from the memory space spanned by them, and
this means that if two different PCI buses are created in the VM, each
with devices under them, they may allocate the same memory space, leading
to PCI Base Address Register which overlap.

This patch series fixes the problem by tracking allocations to child
devices in a separate resource tree, marking them such that the bridge
windows can't overlap.  The main memory resource tree, iomem_resource,
contains resources properly marked as bridge windows, allowing their
children to overlap with them.

Jake Oshins (6):
  drivers:hv: Lock access to hyperv_mmio resource tree
  drivers:hv: Call vmbus_mmio_free() to reverse vmbus_mmio_allocate()
  drivers:hv: Reverse order of resources in hyperv_mmio
  drivers:hv: Track allocations of children of hv_vmbus in private
    resource tree
  drivers:hv: Record MMIO range in use by frame buffer
  drivers:hv: Separate out frame buffer logic when picking MMIO range

 drivers/hv/vmbus_drv.c          | 143 +++++++++++++++++++++++++++++-----------
 drivers/pci/host/pci-hyperv.c   |  14 ++--
 drivers/video/fbdev/hyperv_fb.c |   4 +-
 include/linux/hyperv.h          |   2 +-
 4 files changed, 115 insertions(+), 48 deletions(-)

--
1.9.1

[PATCH v5 1/6] drivers:hv: Lock access to hyperv_mmio resource tree

[Jake Oshins]

In existing code, this tree of resources is created
in single-threaded code and never modified after it is
created, and thus needs no locking.  This patch introduces
a semaphore for tree access, as other patches in this
series introduce run-time modifications of this resource
tree which can happen on multiple threads.

Signed-off-by: Jake Oshins <jakeo@microsoft.com>
---
 drivers/hv/vmbus_drv.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 64713ff..799518b 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -102,6 +102,7 @@ static struct notifier_block hyperv_panic_block = {
 };
 
 struct resource *hyperv_mmio;
+DEFINE_SEMAPHORE(hyperv_mmio_lock);
 
 static int vmbus_exists(void)
 {
@@ -1132,7 +1133,10 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 	resource_size_t range_min, range_max, start, local_min, local_max;
 	const char *dev_n = dev_name(&device_obj->device);
 	u32 fb_end = screen_info.lfb_base + (screen_info.lfb_size << 1);
-	int i;
+	int i, retval;
+
+	retval = -ENXIO;
+	down(&hyperv_mmio_lock);
 
 	for (iter = hyperv_mmio; iter; iter = iter->sibling) {
 		if ((iter->start >= max) || (iter->end <= min))
@@ -1169,13 +1173,17 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 			for (; start + size - 1 <= local_max; start += align) {
 				*new = request_mem_region_exclusive(start, size,
 								    dev_n);
-				if (*new)
-					return 0;
+				if (*new) {
+					retval = 0;
+					goto exit;
+				}
 			}
 		}
 	}
 
-	return -ENXIO;
+exit:
+	up(&hyperv_mmio_lock);
+	return retval;
 }
 EXPORT_SYMBOL_GPL(vmbus_allocate_mmio);
 
-- 
1.9.1

[PATCH v5 2/6] drivers:hv: Call vmbus_mmio_free() to reverse vmbus_mmio_allocate()

[Jake Oshins]

Existing code just called release_mem_region().  Adding a
wrapper around it allows the more complex range tracking
that is introduced later in this patch series.

Signed-off-by: Jake Oshins <jakeo@microsoft.com>
---
 drivers/hv/vmbus_drv.c          | 15 +++++++++++++++
 drivers/pci/host/pci-hyperv.c   | 14 +++++++-------
 drivers/video/fbdev/hyperv_fb.c |  4 ++--
 include/linux/hyperv.h          |  2 +-
 4 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 799518b..60553c1 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1188,6 +1188,21 @@ exit:
 EXPORT_SYMBOL_GPL(vmbus_allocate_mmio);
 
 /**
+ * vmbus_free_mmio() - Free a memory-mapped I/O range.
+ * @start:		Base address of region to release.
+ * @size:		Size of the range to be allocated
+ *
+ * This function releases anything requested by
+ * vmbus_mmio_allocate().
+ */
+void vmbus_free_mmio(resource_size_t start, resource_size_t size)
+{
+	release_mem_region(start, size);
+
+}
+EXPORT_SYMBOL_GPL(vmbus_free_mmio);
+
+/**
  * vmbus_cpu_number_to_vp_number() - Map CPU to VP.
  * @cpu_number: CPU number in Linux terms
  *
diff --git a/drivers/pci/host/pci-hyperv.c b/drivers/pci/host/pci-hyperv.c
index ed651ba..f2559b6 100644
--- a/drivers/pci/host/pci-hyperv.c
+++ b/drivers/pci/host/pci-hyperv.c
@@ -1795,14 +1795,14 @@ static void hv_pci_free_bridge_windows(struct hv_pcibus_device *hbus)
 
 	if (hbus->low_mmio_space && hbus->low_mmio_res) {
 		hbus->low_mmio_res->flags |= IORESOURCE_BUSY;
-		release_mem_region(hbus->low_mmio_res->start,
-				   resource_size(hbus->low_mmio_res));
+		vmbus_free_mmio(hbus->low_mmio_res->start,
+				resource_size(hbus->low_mmio_res));
 	}
 
 	if (hbus->high_mmio_space && hbus->high_mmio_res) {
 		hbus->high_mmio_res->flags |= IORESOURCE_BUSY;
-		release_mem_region(hbus->high_mmio_res->start,
-				   resource_size(hbus->high_mmio_res));
+		vmbus_free_mmio(hbus->high_mmio_res->start,
+				resource_size(hbus->high_mmio_res));
 	}
 }
 
@@ -1880,8 +1880,8 @@ static int hv_pci_allocate_bridge_windows(struct hv_pcibus_device *hbus)
 
 release_low_mmio:
 	if (hbus->low_mmio_res) {
-		release_mem_region(hbus->low_mmio_res->start,
-				   resource_size(hbus->low_mmio_res));
+		vmbus_free_mmio(hbus->low_mmio_res->start,
+				resource_size(hbus->low_mmio_res));
 	}
 
 	return ret;
@@ -1924,7 +1924,7 @@ static int hv_allocate_config_window(struct hv_pcibus_device *hbus)
 
 static void hv_free_config_window(struct hv_pcibus_device *hbus)
 {
-	release_mem_region(hbus->mem_config->start, PCI_CONFIG_MMIO_LENGTH);
+	vmbus_free_mmio(hbus->mem_config->start, PCI_CONFIG_MMIO_LENGTH);
 }
 
 /**
diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
index e2451bd..2fd49b2 100644
--- a/drivers/video/fbdev/hyperv_fb.c
+++ b/drivers/video/fbdev/hyperv_fb.c
@@ -743,7 +743,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
 err3:
 	iounmap(fb_virt);
 err2:
-	release_mem_region(par->mem->start, screen_fb_size);
+	vmbus_free_mmio(par->mem->start, screen_fb_size);
 	par->mem = NULL;
 err1:
 	if (!gen2vm)
@@ -758,7 +758,7 @@ static void hvfb_putmem(struct fb_info *info)
 	struct hvfb_par *par = info->par;
 
 	iounmap(info->screen_base);
-	release_mem_region(par->mem->start, screen_fb_size);
+	vmbus_free_mmio(par->mem->start, screen_fb_size);
 	par->mem = NULL;
 }
 
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index aa0fadc..ecd81c3 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1091,7 +1091,7 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 			resource_size_t min, resource_size_t max,
 			resource_size_t size, resource_size_t align,
 			bool fb_overlap_ok);
-
+void vmbus_free_mmio(resource_size_t start, resource_size_t size);
 int vmbus_cpu_number_to_vp_number(int cpu_number);
 u64 hv_do_hypercall(u64 control, void *input, void *output);
 
-- 
1.9.1

Re: [PATCH v5 2/6] drivers:hv: Call vmbus_mmio_free() to reverse vmbus_mmio_allocate()

[Bjorn Helgaas]

On Tue, Apr 05, 2016 at 06:18:18PM -0700, Jake Oshins wrote:
> Existing code just called release_mem_region().  Adding a
> wrapper around it allows the more complex range tracking
> that is introduced later in this patch series.
> 
> Signed-off-by: Jake Oshins <jakeo@microsoft.com>

With typo fix below,

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

This is mostly non-PCI, so I assume somebody else will merge this series.

> ---
>  drivers/hv/vmbus_drv.c          | 15 +++++++++++++++
>  drivers/pci/host/pci-hyperv.c   | 14 +++++++-------
>  drivers/video/fbdev/hyperv_fb.c |  4 ++--
>  include/linux/hyperv.h          |  2 +-
>  4 files changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 799518b..60553c1 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1188,6 +1188,21 @@ exit:
>  EXPORT_SYMBOL_GPL(vmbus_allocate_mmio);
>  
>  /**
> + * vmbus_free_mmio() - Free a memory-mapped I/O range.
> + * @start:		Base address of region to release.
> + * @size:		Size of the range to be allocated
> + *
> + * This function releases anything requested by
> + * vmbus_mmio_allocate().

s/vmbus_mmio_allocate/vmbus_allocate_mmio/, I think.

> + */
> +void vmbus_free_mmio(resource_size_t start, resource_size_t size)
> +{
> +	release_mem_region(start, size);
> +
> +}
> +EXPORT_SYMBOL_GPL(vmbus_free_mmio);
> +
> +/**
>   * vmbus_cpu_number_to_vp_number() - Map CPU to VP.
>   * @cpu_number: CPU number in Linux terms
>   *
> diff --git a/drivers/pci/host/pci-hyperv.c b/drivers/pci/host/pci-hyperv.c
> index ed651ba..f2559b6 100644
> --- a/drivers/pci/host/pci-hyperv.c
> +++ b/drivers/pci/host/pci-hyperv.c
> @@ -1795,14 +1795,14 @@ static void hv_pci_free_bridge_windows(struct hv_pcibus_device *hbus)
>  
>  	if (hbus->low_mmio_space && hbus->low_mmio_res) {
>  		hbus->low_mmio_res->flags |= IORESOURCE_BUSY;
> -		release_mem_region(hbus->low_mmio_res->start,
> -				   resource_size(hbus->low_mmio_res));
> +		vmbus_free_mmio(hbus->low_mmio_res->start,
> +				resource_size(hbus->low_mmio_res));
>  	}
>  
>  	if (hbus->high_mmio_space && hbus->high_mmio_res) {
>  		hbus->high_mmio_res->flags |= IORESOURCE_BUSY;
> -		release_mem_region(hbus->high_mmio_res->start,
> -				   resource_size(hbus->high_mmio_res));
> +		vmbus_free_mmio(hbus->high_mmio_res->start,
> +				resource_size(hbus->high_mmio_res));
>  	}
>  }
>  
> @@ -1880,8 +1880,8 @@ static int hv_pci_allocate_bridge_windows(struct hv_pcibus_device *hbus)
>  
>  release_low_mmio:
>  	if (hbus->low_mmio_res) {
> -		release_mem_region(hbus->low_mmio_res->start,
> -				   resource_size(hbus->low_mmio_res));
> +		vmbus_free_mmio(hbus->low_mmio_res->start,
> +				resource_size(hbus->low_mmio_res));
>  	}
>  
>  	return ret;
> @@ -1924,7 +1924,7 @@ static int hv_allocate_config_window(struct hv_pcibus_device *hbus)
>  
>  static void hv_free_config_window(struct hv_pcibus_device *hbus)
>  {
> -	release_mem_region(hbus->mem_config->start, PCI_CONFIG_MMIO_LENGTH);
> +	vmbus_free_mmio(hbus->mem_config->start, PCI_CONFIG_MMIO_LENGTH);
>  }
>  
>  /**
> diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
> index e2451bd..2fd49b2 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -743,7 +743,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
>  err3:
>  	iounmap(fb_virt);
>  err2:
> -	release_mem_region(par->mem->start, screen_fb_size);
> +	vmbus_free_mmio(par->mem->start, screen_fb_size);
>  	par->mem = NULL;
>  err1:
>  	if (!gen2vm)
> @@ -758,7 +758,7 @@ static void hvfb_putmem(struct fb_info *info)
>  	struct hvfb_par *par = info->par;
>  
>  	iounmap(info->screen_base);
> -	release_mem_region(par->mem->start, screen_fb_size);
> +	vmbus_free_mmio(par->mem->start, screen_fb_size);
>  	par->mem = NULL;
>  }
>  
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index aa0fadc..ecd81c3 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1091,7 +1091,7 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
>  			resource_size_t min, resource_size_t max,
>  			resource_size_t size, resource_size_t align,
>  			bool fb_overlap_ok);
> -
> +void vmbus_free_mmio(resource_size_t start, resource_size_t size);
>  int vmbus_cpu_number_to_vp_number(int cpu_number);
>  u64 hv_do_hypercall(u64 control, void *input, void *output);
>  
> -- 
> 1.9.1
> 

[PATCH v5 3/6] drivers:hv: Reverse order of resources in hyperv_mmio

[Jake Oshins]

A patch later in this series allocates child nodes
in this resource tree.  For that to work, this tree
needs to be sorted in ascending order.

Signed-off-by: Jake Oshins <jakeo@microsoft.com>
---
 drivers/hv/vmbus_drv.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 60553c1..1ce47d0 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1049,7 +1049,6 @@ static acpi_status vmbus_walk_resources(struct acpi_resource *res, void *ctx)
 	new_res->end = end;
 
 	/*
-	 * Stick ranges from higher in address space at the front of the list.
 	 * If two ranges are adjacent, merge them.
 	 */
 	do {
@@ -1070,7 +1069,7 @@ static acpi_status vmbus_walk_resources(struct acpi_resource *res, void *ctx)
 			break;
 		}
 
-		if ((*old_res)->end < new_res->start) {
+		if ((*old_res)->start > new_res->end) {
 			new_res->sibling = *old_res;
 			if (prev_res)
 				(*prev_res)->sibling = new_res;
-- 
1.9.1

[PATCH v5 4/6] drivers:hv: Track allocations of children of hv_vmbus in private resource tree

[Jake Oshins]

This patch changes vmbus_allocate_mmio() and vmbus_free_mmio() so
that when child paravirtual devices allocate memory-mapped I/O
space, they allocate it privately from a resource tree pointed
at by hyperv_mmio and also by the public resource tree
iomem_resource.  This allows the region to be marked as "busy"
in the private tree, but a "bridge window" in the public tree,
guaranteeing that no two bridge windows will overlap each other
but while also allowing the PCI device children of the bridge
windows to overlap that window.

One might conclude that this belongs in the pnp layer, rather
than in this driver.  Rafael Wysocki, the maintainter of the
pnp layer, has previously asked that we not modify the pnp layer
as it is considered deprecated.  This patch is thus essentially
a workaround.

Signed-off-by: Jake Oshins <jakeo@microsoft.com>
---
 drivers/hv/vmbus_drv.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 1ce47d0..dfc6149 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1128,7 +1128,7 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 			resource_size_t size, resource_size_t align,
 			bool fb_overlap_ok)
 {
-	struct resource *iter;
+	struct resource *iter, *shadow;
 	resource_size_t range_min, range_max, start, local_min, local_max;
 	const char *dev_n = dev_name(&device_obj->device);
 	u32 fb_end = screen_info.lfb_base + (screen_info.lfb_size << 1);
@@ -1170,12 +1170,22 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 
 			start = (local_min + align - 1) & ~(align - 1);
 			for (; start + size - 1 <= local_max; start += align) {
+				shadow = __request_region(iter, start,
+							  size,
+							  NULL,
+							  IORESOURCE_BUSY);
+				if (!shadow)
+					continue;
+
 				*new = request_mem_region_exclusive(start, size,
 								    dev_n);
 				if (*new) {
+					shadow->name = (char *)*new;
 					retval = 0;
 					goto exit;
 				}
+
+				__release_region(iter, start, size);
 			}
 		}
 	}
@@ -1196,7 +1206,17 @@ EXPORT_SYMBOL_GPL(vmbus_allocate_mmio);
  */
 void vmbus_free_mmio(resource_size_t start, resource_size_t size)
 {
+	struct resource *iter;
+
+	down(&hyperv_mmio_lock);
+	for (iter = hyperv_mmio; iter; iter = iter->sibling) {
+		if ((iter->start >= start + size) || (iter->end <= start))
+			continue;
+
+		__release_region(iter, start, size);
+	}
 	release_mem_region(start, size);
+	up(&hyperv_mmio_lock);
 
 }
 EXPORT_SYMBOL_GPL(vmbus_free_mmio);
-- 
1.9.1

[PATCH v5 5/6] drivers:hv: Record MMIO range in use by frame buffer

[Jake Oshins]

Later in the boot sequence, we need to figure out which memory
ranges can be given out to various paravirtual drivers.  The
hyperv_fb driver should, ideally, be placed right on top of
the frame buffer, without some other device getting plopped on
top of this range in the meantime.  Recording this now allows
that to be guaranteed.

Signed-off-by: Jake Oshins <jakeo@microsoft.com>
---
 drivers/hv/vmbus_drv.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index dfc6149..eaa5c3b 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -41,6 +41,7 @@
 #include <linux/ptrace.h>
 #include <linux/screen_info.h>
 #include <linux/kdebug.h>
+#include <linux/efi.h>
 #include "hyperv_vmbus.h"
 
 static struct acpi_device  *hv_acpi_dev;
@@ -101,6 +102,8 @@ static struct notifier_block hyperv_panic_block = {
 	.notifier_call = hyperv_panic_event,
 };
 
+static const char *fb_mmio_name = "fb_range";
+static struct resource *fb_mmio;
 struct resource *hyperv_mmio;
 DEFINE_SEMAPHORE(hyperv_mmio_lock);
 
@@ -1091,6 +1094,12 @@ static int vmbus_acpi_remove(struct acpi_device *device)
 	struct resource *next_res;
 
 	if (hyperv_mmio) {
+		if (fb_mmio) {
+			__release_region(hyperv_mmio, fb_mmio->start,
+					 resource_size(fb_mmio));
+			fb_mmio = NULL;
+		}
+
 		for (cur_res = hyperv_mmio; cur_res; cur_res = next_res) {
 			next_res = cur_res->sibling;
 			kfree(cur_res);
@@ -1100,6 +1109,30 @@ static int vmbus_acpi_remove(struct acpi_device *device)
 	return 0;
 }
 
+static void vmbus_reserve_fb(void)
+{
+	int size;
+	/*
+	 * Make a claim for the frame buffer in the resource tree under the
+	 * first node, which will be the one below 4GB.  The length seems to
+	 * be underreported, particularly in a Generation 1 VM.  So start out
+	 * reserving a larger area and make it smaller until it succeeds.
+	 */
+
+	if (screen_info.lfb_base) {
+		if (efi_enabled(EFI_BOOT))
+			size = max_t(__u32, screen_info.lfb_size, 0x800000);
+		else
+			size = max_t(__u32, screen_info.lfb_size, 0x4000000);
+
+		for (; !fb_mmio && (size >= 0x100000); size >>= 1) {
+			fb_mmio = __request_region(hyperv_mmio,
+						   screen_info.lfb_base, size,
+						   fb_mmio_name, 0);
+		}
+	}
+}
+
 /**
  * vmbus_allocate_mmio() - Pick a memory-mapped I/O range.
  * @new:		If successful, supplied a pointer to the
@@ -1261,8 +1294,10 @@ static int vmbus_acpi_add(struct acpi_device *device)
 
 		if (ACPI_FAILURE(result))
 			continue;
-		if (hyperv_mmio)
+		if (hyperv_mmio) {
+			vmbus_reserve_fb();
 			break;
+		}
 	}
 	ret_val = 0;
 
-- 
1.9.1

[PATCH v5 6/6] drivers:hv: Separate out frame buffer logic when picking MMIO range

[Jake Oshins]

Simplify the logic that picks MMIO ranges by pulling out the
logic related to trying to lay frame buffer claim on top of where
the firmware placed the frame buffer.

Signed-off-by: Jake Oshins <jakeo@microsoft.com>
---
 drivers/hv/vmbus_drv.c | 80 ++++++++++++++++++++++----------------------------
 1 file changed, 35 insertions(+), 45 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index eaa5c3b..a29a6c0 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1162,64 +1162,54 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 			bool fb_overlap_ok)
 {
 	struct resource *iter, *shadow;
-	resource_size_t range_min, range_max, start, local_min, local_max;
+	resource_size_t range_min, range_max, start;
 	const char *dev_n = dev_name(&device_obj->device);
-	u32 fb_end = screen_info.lfb_base + (screen_info.lfb_size << 1);
-	int i, retval;
+	int retval;
 
 	retval = -ENXIO;
 	down(&hyperv_mmio_lock);
 
+	/*
+	 * If overlaps with frame buffers are allowed, then first attempt to
+	 * make the allocation from within the reserved region.  Because it
+	 * is already reserved, no shadow allocation is necessary.
+	 */
+	if (fb_overlap_ok && fb_mmio && !(min > fb_mmio->end) &&
+	    !(max < fb_mmio->start)) {
+
+		range_min = fb_mmio->start;
+		range_max = fb_mmio->end;
+		start = (range_min + align - 1) & ~(align - 1);
+		for (; start + size - 1 <= range_max; start += align) {
+			*new = request_mem_region_exclusive(start, size, dev_n);
+			if (*new) {
+				retval = 0;
+				goto exit;
+			}
+		}
+	}
+
 	for (iter = hyperv_mmio; iter; iter = iter->sibling) {
 		if ((iter->start >= max) || (iter->end <= min))
 			continue;
 
 		range_min = iter->start;
 		range_max = iter->end;
-
-		/* If this range overlaps the frame buffer, split it into
-		   two tries. */
-		for (i = 0; i < 2; i++) {
-			local_min = range_min;
-			local_max = range_max;
-			if (fb_overlap_ok || (range_min >= fb_end) ||
-			    (range_max <= screen_info.lfb_base)) {
-				i++;
-			} else {
-				if ((range_min <= screen_info.lfb_base) &&
-				    (range_max >= screen_info.lfb_base)) {
-					/*
-					 * The frame buffer is in this window,
-					 * so trim this into the part that
-					 * preceeds the frame buffer.
-					 */
-					local_max = screen_info.lfb_base - 1;
-					range_min = fb_end;
-				} else {
-					range_min = fb_end;
-					continue;
-				}
+		start = (range_min + align - 1) & ~(align - 1);
+		for (; start + size - 1 <= range_max; start += align) {
+			shadow = __request_region(iter, start, size, NULL,
+						  IORESOURCE_BUSY);
+			if (!shadow)
+				continue;
+
+			*new = request_mem_region_exclusive(start, size, dev_n);
+			if (*new) {
+				shadow->name = (char *)*new;
+				retval = 0;
+				goto exit;
 			}
 
-			start = (local_min + align - 1) & ~(align - 1);
-			for (; start + size - 1 <= local_max; start += align) {
-				shadow = __request_region(iter, start,
-							  size,
-							  NULL,
-							  IORESOURCE_BUSY);
-				if (!shadow)
-					continue;
-
-				*new = request_mem_region_exclusive(start, size,
-								    dev_n);
-				if (*new) {
-					shadow->name = (char *)*new;
-					retval = 0;
-					goto exit;
-				}
-
-				__release_region(iter, start, size);
-			}
+			__release_region(iter, start, size);
 		}
 	}
 
-- 
1.9.1