+ intel-gfx and dri-devel Hi, This is the Gvt-g local display direct flip discussion. And please reference here (https://lists.freedesktop.org/archives/intel-gvt-dev/2018-December/004594.html) to see what the feature is. Mostly, with Gvt-g local display direct flip feature, vGPU can directly post its framebuffer to the assigned local HW display plane w/o host userspace involvement. We propose to use drm_framebuffer to stand for the framebuffer attached on a vGPU's plane. Since GVT-g can guarantee this drm_framebuffer is updated with the latest info of the framebuffer attached on the vGPU's plane during guest page flip, atomic async plane update mechanism is used in i915 to help updating the vGPU assigned HW planes efficiently with those special drm_framebuffers. Host userspace is in charge of the HW plane assignment. And currently we are discussing about: 1) Whether the drm_framebuffer can be generic enough to deal with the direct flip use cases? In our case, the drm_framebuffer has no gem backends, as the display memory management is handled by guest. 2) How to expose the drm_framebuffer to the host userspace? a) Through vfio/display ioctl The kernel implementation is easy. But this might turn qemu into a display manager. Qemu has to handle all the KMS mode-setting stuff things. b) Through drm/i915 ioctl This needs some code change in kernel space to deal with this ioctl between i915 and GVT-g. But the good thing is this ioctl can be used by the userspace display manager w/o vfio/display's help. Thanks. > -----Original Message----- > From: Gerd Hoffmann [mailto:kraxel@xxxxxxxxxx] > Sent: Thursday, December 20, 2018 7:34 PM > To: Zhang, Tina <tina.zhang@xxxxxxxxx> > Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; Wang, Zhenyu Z > <zhenyu.z.wang@xxxxxxxxx>; Wang, Zhi A <zhi.a.wang@xxxxxxxxx>; He, Min > <min.he@xxxxxxxxx>; Yuan, Hang <hang.yuan@xxxxxxxxx>; Alex Williamson > <alex.williamson@xxxxxxxxxx>; Lv, Zhiyuan <zhiyuan.lv@xxxxxxxxx>; Vetter, > Daniel <daniel.vetter@xxxxxxxxx>; intel-gvt-dev <intel-gvt- > dev@xxxxxxxxxxxxxxxxxxxxx>; Wang, Hongbo <hongbo.wang@xxxxxxxxx> > Subject: Re: Local Display Direct Flip Feature Discussion > > On Thu, Dec 20, 2018 at 08:45:09AM +0000, Zhang, Tina wrote: > > > > > > > -----Original Message----- > > > From: intel-gvt-dev > > > [mailto:intel-gvt-dev-bounces@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of > > > Gerd Hoffmann > > > Sent: Wednesday, December 19, 2018 7:26 PM > > > To: Zhang, Tina <tina.zhang@xxxxxxxxx> > > > Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; Wang, Zhenyu Z > > > <zhenyu.z.wang@xxxxxxxxx>; Wang, Zhi A <zhi.a.wang@xxxxxxxxx>; He, > > > Min <min.he@xxxxxxxxx>; Yuan, Hang <hang.yuan@xxxxxxxxx>; Alex > > > Williamson <alex.williamson@xxxxxxxxxx>; Lv, Zhiyuan > > > <zhiyuan.lv@xxxxxxxxx>; Vetter, Daniel <daniel.vetter@xxxxxxxxx>; > > > intel-gvt-dev <intel-gvt- dev@xxxxxxxxxxxxxxxxxxxxx>; Wang, Hongbo > > > <hongbo.wang@xxxxxxxxx> > > > Subject: Re: Local Display Direct Flip Feature Discussion > > > > > > Hi, > > > > > > > > Isn't a framebuffer just a gem object with metadata (fourcc, > > > > > width, height, stride, ...) attached? So I'm wondering how that > > > > > works in detail. What happens on page-flip? Do you make the > > > > > framebuffer reference another gem object then? Or do you blit > > > > > the guest display to the framebuffer? > > > > The special DRM framebuffer is transparent to the guest display driver. > > > > We just want to use this object to save the guest framebuffer info > > > > in host-side. > > > > > > Hmm, so this object isn't a normal drm framebuffer. You are just > > > masquerading it as drm framebuffer, so you can assign it to a drm > > > crtc or drm plane. > > Not so bad actually :-) > > Host just wants to use a drm framebuffer to describe the drm > > framebuffer attached on a vGPU's plane. And the only special thing is > > that this host drm frambuffer has on gem backends, which means host > > cannot manage the > > ... has no gem ... ? > > > memory of this drm framebuffer. And it's OK, as the guest does the > management. > > Besides, generic drm framebuffer was designed to be able to deal with > > this kind of situation. > > Hmm, ok. Never dealed with framebuffers not backed by gem objects so far, > seems to not be very common too. But the doc comments in > include/drm/drm_framebuffer.h suggest it is fine indeed. > > What is the plan for hardware cursor support? Support two framebuffers, for > primary and cursor? We got the hardware cursor support in our to-go list: https://lists.freedesktop.org/archives/intel-gvt-dev/2018-December/004559.html We will support both primary and cursor. > > > > It makes sense to allow mapping guest outputs to host outputs. I > > > think it is more useful to handle that at crtc level not plane > > > level, so it'll work for both primary and cursor plane. I think it > > > would be cleaner to introduce new drm (generic or i915) APIs for > > > that instead of creating special framebuffer objects which behave in non- > standard ways. > > The APIs solution is one of our options and we have patch for it. The > > userspace interface is still under discussion. And here are the three > candidates: > > 1) Through i915 ioctl > > 2) Through vfio/display ioctl > > 3) Through GVT-g sys fs or debugfs > > And option2 is considered as the preferred one, as this drm > > framebuffer is related to the vGPU. > > (3) Having this (in debugfs) would be useful for debugging purposes, > even in addition to (1) or (2). > > (2) Implies qemu must handle it, so support must either be implemented > in qemu directly, or in another process cooperating with qemu in > some way (extending spice protocol & spice client comes to mind). > Given that the input side (mouse and kbd events) need cooperation > with qemu anyway this might not be much of a limitation though. > > (1) Would allow to handle this without having to worry about qemu/vfio > at all. Needs some infrastructure (drm ioctl to enumerate vgpus for > example). Possibly the amdgpu guys (which are doing vgpu using > sr/iov instead of mdev) are interested in this too. > > No clear winner, but I tend to agree that (2) looks best. > > > > Alternatively try to tackle the problem in a completely different way. > > > Right now qemu will check for display changes using a timer. We > > > could add some page flip notification mechanism (using an eventfd > > > for example) so qemu can update the host plane (or notify spice > > > client) instantly instead of waiting for the next display refresh timer tick. > > We could do that. But the local display direct flip feature doesn't > > want to involve the host userspace, considering the performance. > > Is the additional switch to qemu actually that much of a performance > problem? I somehow doubt it, it's not like we are doing hundreds of page- > flips per second. If we only focus on the vblank events (not page flip events), I think it might be OK, as the vblank event is per crtc not per plane. I think we can use dma-buf to have a try. Thanks. BR, Tina > > > Maybe we can add this mechanism to dma-buf use case. > > That would be cool. > > cheers, > Gerd _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
