vt: add a vt_dummy driver
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Authored by corvink on Mar 27 2023, 10:11 AM.

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Reviewers

jhb
manu
markj
dumbbell

Summary

By default vt uses vt_efifb if no video driver is loaded. vt_efifb grabs
the EFI framebuffer and writes directly to it. When the graphics driver
is unloaded from the GPU, vt_efifb still writes to the EFI framebuffer.
If GPU passthrough for a bhyve VM is used, the guest uses the EFI
framebuffer too. When vt_efifb writes new data to the EFI framebuffer it
results in stripes on the guest display. This can be avoided by loading
this new vt_dummy driver that ignores all draw calls.

Note: As vt_dummy ignores all draw call, loading it will freeze the
display.

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rG FreeBSD src repository

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Build Status

Buildable 50598
Build 47489: arc lint + arc unit

Event Timeline

corvink created this revision.Mar 27 2023, 10:11 AM

Herald added a subscriber: imp. · View Herald TranscriptMar 27 2023, 10:11 AM

corvink requested review of this revision.Mar 27 2023, 10:11 AM

Harbormaster completed remote builds in B50597: Diff 119501.Mar 27 2023, 10:11 AM

guest-patmaddox added a subscriber: guest-patmaddox.Mar 27 2023, 10:16 AM

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remove obsolete __FBSDID

Harbormaster completed remote builds in B50598: Diff 119502.Mar 27 2023, 10:17 AM

Unsure if this is the best approach so I have a few questions :

- What happens if you kldload vt_dummy and then kldunload vt_dummy ? Do you get back efifb ?
Wouldn't setting FB_FLAG_NOWRITE somewhere in bhyve work too ? (We would need some vt function that will freeze/unfreeze the current vt)

Adding @dumbbell in case he have some ideas.

manu added a reviewer: dumbbell.Mar 27 2023, 11:16 AM

guest-patmaddox removed a subscriber: guest-patmaddox.Mar 27 2023, 11:35 AM

In D39283#894397, @manu wrote:

Unsure if this is the best approach so I have a few questions :

What happens if you kldload vt_dummy and then kldunload vt_dummy ? Do you get back efifb ?

Yes, it get's back to efifb without any issues.

Wouldn't setting FB_FLAG_NOWRITE somewhere in bhyve work too ? (We would need some vt function that will freeze/unfreeze the current vt)

Looks like setting FB_FLAG_NOWRITE could be sufficient. There's no interface to set it yet. I'm going to test it.

Yeah, I solved the same problem in drm initially by writing a dummy driver too, but now it's been replaced by FB_FLAG_NOWRITE.
This is how we're doing it, it's a bit annoying that we have to check the driver and reach into its softc, ideally flags like that would be a more outer-level thing somehow (?)

In D39283#895490, @val_packett.cool wrote:

Yeah, I solved the same problem in drm initially by writing a dummy driver too, but now it's been replaced by FB_FLAG_NOWRITE.
This is how we're doing it, it's a bit annoying that we have to check the driver and reach into its softc, ideally flags like that would be a more outer-level thing somehow (?)

Yes we clearly need some api to freeze/unfreeze the current vt now that we have two use case. And also something for userland (and maybe hooked to vidcontrol somehow ?)

In D39283#895533, @manu wrote:

In D39283#895490, @val_packett.cool wrote:

Yeah, I solved the same problem in drm initially by writing a dummy driver too, but now it's been replaced by FB_FLAG_NOWRITE.
This is how we're doing it, it's a bit annoying that we have to check the driver and reach into its softc, ideally flags like that would be a more outer-level thing somehow (?)

Yes we clearly need some api to freeze/unfreeze the current vt now that we have two use case. And also something for userland (and maybe hooked to vidcontrol somehow ?)