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>
