RE: [PATCH v2 3/3] Drivers: hv: Never allocate anything besides framebuffer from framebuffer memory region

[Michael Kelley (LINUX)]

From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Thursday, August 25, 2022 2:00 AM
> 
> Passed through PCI device sometimes misbehave on Gen1 VMs when Hyper-V
> DRM driver is also loaded. Looking at IOMEM assignment, we can see e.g.
> 
> $ cat /proc/iomem
> ...
> f8000000-fffbffff : PCI Bus 0000:00
>   f8000000-fbffffff : 0000:00:08.0
>     f8000000-f8001fff : bb8c4f33-2ba2-4808-9f7f-02f3b4da22fe
> ...
> fe0000000-fffffffff : PCI Bus 0000:00
>   fe0000000-fe07fffff : bb8c4f33-2ba2-4808-9f7f-02f3b4da22fe
>     fe0000000-fe07fffff : 2ba2:00:02.0
>       fe0000000-fe07fffff : mlx4_core
> 
> the interesting part is the 'f8000000' region as it is actually the
> VM's framebuffer:
> 
> $ lspci -v
> ...
> 0000:00:08.0 VGA compatible controller: Microsoft Corporation Hyper-V virtual VGA
> (prog-if 00 [VGA controller])
> 	Flags: bus master, fast devsel, latency 0, IRQ 11
> 	Memory at f8000000 (32-bit, non-prefetchable) [size=64M]
> ...
> 
>  hv_vmbus: registering driver hyperv_drm
>  hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm] Synthvid Version major 3, minor 5
>  hyperv_drm 0000:00:08.0: vgaarb: deactivate vga console
>  hyperv_drm 0000:00:08.0: BAR 0: can't reserve [mem 0xf8000000-0xfbffffff]
>  hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm] Cannot request framebuffer, boot fb still active?
> 
> Note: "Cannot request framebuffer" is not a fatal error in
> hyperv_setup_gen1() as the code assumes there's some other framebuffer
> device there but we actually have some other PCI device (mlx4 in this
> case) config space there!

My apologies for not getting around to commenting on the previous
version of this patch.  The function hyperv_setup_gen1() and the
"Cannot request framebuffer" message have gone away as of
commit a0ab5abced55.

> 
> The problem appears to be that vmbus_allocate_mmio() can allocate from
> the reserved framebuffer region (fb_overlap_ok), however, if the
> request to allocate MMIO comes from some other device before
> framebuffer region is taken, it can happily use framebuffer region for
> it. 

Interesting. I had never looked at the details of vmbus_allocate_mmio().
The semantics one might assume of a parameter named "fb_overlap_ok"
aren't implemented because !fb_overlap_ok essentially has no effect.   The
existing semantics are really "prefer_fb_overlap".  This patch implements
the expected and needed semantics, which is to not allocate from the frame
buffer space when !fb_overlap_ok.

If that's an accurate high level summary, maybe this commit message
could describe it that way?  The other details you provide about what can
go wrong should still be included as well.

> Note, Gen2 VMs are usually unaffected by the issue because
> framebuffer region is already taken by EFI fb (in case kernel supports
> it) but Gen1 VMs may have this region unclaimed by the time Hyper-V PCI
> pass-through driver tries allocating MMIO space if Hyper-V DRM/FB drivers
> load after it. Devices can be brought up in any sequence so let's
> resolve the issue by always ignoring 'fb_mmio' region for non-FB
> requests, even if the region is unclaimed.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  drivers/hv/vmbus_drv.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 536f68e563c6..3c833ea60db6 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -2331,7 +2331,7 @@ 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;
> +	resource_size_t range_min, range_max, start, end;
>  	const char *dev_n = dev_name(&device_obj->device);
>  	int retval;
> 
> @@ -2366,6 +2366,14 @@ int vmbus_allocate_mmio(struct resource **new, struct
> hv_device *device_obj,
>  		range_max = iter->end;
>  		start = (range_min + align - 1) & ~(align - 1);
>  		for (; start + size - 1 <= range_max; start += align) {
> +			end = start + size - 1;
> +
> +			/* Skip the whole fb_mmio region if not fb_overlap_ok */
> +			if (!fb_overlap_ok && fb_mmio &&
> +			    (((start >= fb_mmio->start) && (start <= fb_mmio->end)) ||
> +			     ((end >= fb_mmio->start) && (end <= fb_mmio->end))))
> +				continue;
> +
>  			shadow = __request_region(iter, start, size, NULL,
>  						  IORESOURCE_BUSY);
>  			if (!shadow)
> --
> 2.37.1

Other than my musings on the commit message,

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>