Re: [Mesa-dev] RFC - libglvnd and GLXVND vendor enumeration to facilitate GLX multi-vendor PRIME GPU offload
On 2/13/19 2:32 PM, Andy Ritger wrote:
On Wed, Feb 13, 2019 at 12:15:02PM -0700, Kyle Brenneman wrote:
On 02/12/2019 01:58 AM, Michel Dänzer wrote:
On 2019-02-11 5:18 p.m., Andy Ritger wrote:
On Mon, Feb 11, 2019 at 12:09:26PM +0100, Michel Dänzer wrote:
On 2019-02-08 11:43 p.m., Kyle Brenneman wrote:
Also, is Mesa the only client-side vendor library that works with the
Xorg GLX module? I vaguely remember that there was at least one other
driver that did, but I don't remember the details anymore.
AFAIK, the amdgpu-pro OpenGL driver can work with the Xorg GLX module
(or its own forked version of it).
Maybe the amdgpu-pro OpenGL driver uses a fork of the Xorg GLX module
(or sets the "GlxVendorLibrary" X configuration option?), but it doesn't
look to me like the in-tree Xorg GLX module could report anything other
than "mesa" for GLX_VENDOR_NAMES_EXT, without custom user configuration.
GLX_VENDOR_NAMES_EXT, which client-side glvnd uses to pick the
libGLX_${vendor}.so to load, is implemented in the Xorg GLX module
with this:
xserver/glx/glxcmds.c:__glXDisp_QueryServerString():
case GLX_VENDOR_NAMES_EXT:
if (pGlxScreen->glvnd) {
ptr = pGlxScreen->glvnd;
break;
}
pGlxScreen->glvnd appears to be assigned here, defaulting to "mesa",
though allowing an xorg.conf override via the "GlxVendorLibrary" option:
xserver/glx/glxdri2.c:__glXDRIscreenProbe():
xf86ProcessOptions(pScrn->scrnIndex, pScrn->options, options);
glvnd = xf86GetOptValString(options, GLXOPT_VENDOR_LIBRARY);
if (glvnd)
screen->base.glvnd = xnfstrdup(glvnd);
free(options);
if (!screen->base.glvnd)
screen->base.glvnd = strdup("mesa");
And swrast unconditionally sets pGlxScreen->glvnd to "mesa":
xserver/glx/glxdriswrast.c:__glXDRIscreenProbe():
screen->base.glvnd = strdup("mesa");
Is there more to this that I'm missing?
I don't think so, I suspect we were just assuming slightly different
definitions of "works". :)
That should get fixed, but since that applies to the libglvnd's normal
vendor selection, I'd say it's orthogonal to GPU offloading. Off the top of
my head, the "GlxVendorLibrary" option ought to work regardless of which
__GLXprovider it finds. I think it would be possible to add a function to
let a driver override the GLX_VENDOR_NAMES_EXT string, too.
I think the point, though, is that thus far, libGLX_mesa.so is the only
glvnd client-side GLX implementation that will be loaded for use with
Xorg's GLX. Thus, it doesn't seem to refute ajax's comment from earlier
in the thread:
I don't see that those are related. The GLX_VENDOR_NAMES_EXT string
tells libglvnd which vendor to use by default. GPU offloading, more or
less by definition, means using something other than the default.
At the other extreme, the server could do nearly all the work of
generating the possible __GLX_VENDOR_LIBRARY_NAME strings (with the
practical downside of each server-side GLX vendor needing to enumerate
the GPUs it can drive, in order to generate the hardware-specific
identifiers).
I don't think this downside is much of a burden? If you're registering
a provider other than Xorg's you're already doing it from the DDX
driver
-Kyle
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Re: [Mesa-dev] RFC - libglvnd and GLXVND vendor enumeration to facilitate GLX multi-vendor PRIME GPU offload
On 02/12/2019 01:58 AM, Michel Dänzer wrote:
On 2019-02-11 5:18 p.m., Andy Ritger wrote:
On Mon, Feb 11, 2019 at 12:09:26PM +0100, Michel Dänzer wrote:
On 2019-02-08 11:43 p.m., Kyle Brenneman wrote:
Also, is Mesa the only client-side vendor library that works with the
Xorg GLX module? I vaguely remember that there was at least one other
driver that did, but I don't remember the details anymore.
AFAIK, the amdgpu-pro OpenGL driver can work with the Xorg GLX module
(or its own forked version of it).
Maybe the amdgpu-pro OpenGL driver uses a fork of the Xorg GLX module
(or sets the "GlxVendorLibrary" X configuration option?), but it doesn't
look to me like the in-tree Xorg GLX module could report anything other
than "mesa" for GLX_VENDOR_NAMES_EXT, without custom user configuration.
GLX_VENDOR_NAMES_EXT, which client-side glvnd uses to pick the
libGLX_${vendor}.so to load, is implemented in the Xorg GLX module
with this:
xserver/glx/glxcmds.c:__glXDisp_QueryServerString():
case GLX_VENDOR_NAMES_EXT:
if (pGlxScreen->glvnd) {
ptr = pGlxScreen->glvnd;
break;
}
pGlxScreen->glvnd appears to be assigned here, defaulting to "mesa",
though allowing an xorg.conf override via the "GlxVendorLibrary" option:
xserver/glx/glxdri2.c:__glXDRIscreenProbe():
xf86ProcessOptions(pScrn->scrnIndex, pScrn->options, options);
glvnd = xf86GetOptValString(options, GLXOPT_VENDOR_LIBRARY);
if (glvnd)
screen->base.glvnd = xnfstrdup(glvnd);
free(options);
if (!screen->base.glvnd)
screen->base.glvnd = strdup("mesa");
And swrast unconditionally sets pGlxScreen->glvnd to "mesa":
xserver/glx/glxdriswrast.c:__glXDRIscreenProbe():
screen->base.glvnd = strdup("mesa");
Is there more to this that I'm missing?
I don't think so, I suspect we were just assuming slightly different
definitions of "works". :)
That should get fixed, but since that applies to the libglvnd's normal
vendor selection, I'd say it's orthogonal to GPU offloading. Off the top
of my head, the "GlxVendorLibrary" option ought to work regardless of
which __GLXprovider it finds. I think it would be possible to add a
function to let a driver override the GLX_VENDOR_NAMES_EXT string, too.
-Kyle
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Re: [Mesa-dev] RFC - libglvnd and GLXVND vendor enumeration to facilitate GLX multi-vendor PRIME GPU offload
On 02/11/2019 02:51 PM, Andy Ritger wrote:
On Fri, Feb 08, 2019 at 03:43:25PM -0700, Kyle Brenneman wrote:
On 2/8/19 2:33 PM, Andy Ritger wrote:
On Fri, Feb 08, 2019 at 03:01:33PM -0500, Adam Jackson wrote:
On Fri, 2019-02-08 at 10:19 -0800, Andy Ritger wrote:
(1) If configured for PRIME GPU offloading (environment variable or
application profile), client-side libglvnd could load the possible
libGLX_${vendor}.so libraries it finds, and call into each to
find which vendor (and possibly which GPU) matches the specified
string. Once a vendor is selected, the vendor library could optionally
tell the X server which GLX vendor to use server-side for this
client connection.
I'm not a huge fan of the "dlopen everything" approach, if it can be
avoided.
Yes, I agree.
I'm pretty sure libglvnd could avoid unnecessarily loading vendor libraries
without adding nearly so much complexity.
If libglvnd just has a list of additional vendor library names to try, then
you could just have a flag to tell libglvnd to check some server string for
that name before it loads the vendor. If a client-side vendor would need a
server-side counterpart to work, then libglvnd can check for that. The
server only needs to keep a list of names to send back, which would be a
trivial (and backward-compatible) addition to the GLXVND interface.
Also, even without that, I don't think the extra dlopen calls would be a
problem in practice. It would only ever happen in applications that are
configured for offloading, which are (more-or-less by definition)
heavy-weight programs, so an extra millisecond or so of startup time is
probably fine.
But why incur that loading if we don't need to?
As I noted, we can still avoid loading extra loads even with an (almost)
strictly client-based design. You don't need to do any sort of
server-based device enumeration, all you need is something in the server
to add a string to a list that the client can query.
But, there's no reason that query can't be optional, and there's no
reason it has to be coupled with anything else.
I think I'd rather have a new enum for GLXQueryServerString
that elaborates on GLX_VENDOR_NAMES_EXT (perhaps GLX_VENDOR_MAP_EXT),
with the returned string a space-delimited list of :.
libGL could accept either a profile or a vendor name in the environment
variable, and the profile can be either semantic like
performance/battery, or a hardware selector, or whatever else.
This would probably be a layered extension, call it GLX_EXT_libglvnd2,
which you'd check for in the (already per-screen) server extension
string before trying to actually use.
That all sounds reasonable to me.
At the other extreme, the server could do nearly all the work of
generating the possible __GLX_VENDOR_LIBRARY_NAME strings (with the
practical downside of each server-side GLX vendor needing to enumerate
the GPUs it can drive, in order to generate the hardware-specific
identifiers).
I don't think this downside is much of a burden? If you're registering
a provider other than Xorg's you're already doing it from the DDX
driver (I think? Are y'all doing that from your libglx instead?), and
when that initializes it already knows which device it's driving.
Right. It will be easy enough for the NVIDIA X driver + NVIDIA server-side GLX.
Kyle and I were chatting about this, and we weren't sure whether people
would object to doing that for the Xorg GLX provider: to create the
hardware names, Xorg's GLX would need to enumerate all the DRM devices
and list them all as possible : pairs for the Xorg
GLX-driven screens. But, now that I look at it more closely, it looks
like drmGetDevices2() would work well for that.
So, if you're not concerned with that burden, I'm not. I'll try coding
up the Xorg GLX part of things and see how it falls into place.
That actually is one of my big concerns: I'd like to come up with something
that can give something equivalent to Mesa's existing DRI_PRIME setting, and
requiring that logic to be in the server seems like a very poor match. You'd
need to take all of the device selection and enumeration stuff from Mesa and
transplant it into the Xorg GLX module, and then you'd need to define some
sort of protocol to get that data back into Mesa where you actually need it.
Or else you need to duplicate it between the client and server, which seems
like the worst of both worlds.
Is this actually a lot of code? I'll try to put together a prototype so
we can see how much it is, but if it is just calling drmGetDevices2() and
then building PCI BusID-based names, that doesn't seem unreasonable to me.
The fact that it's required *at all* tells you that a server-based
design doesn't match the reality of existing drivers. I've also seen
ideas for GLX implementations based on EGL or Vulkan, which probably
wouldn't be able to work with server-side device enumeration.
And like I pointed out, adding that requirement doesn't give you
anything that yo
Re: RFC - libglvnd and GLXVND vendor enumeration to facilitate GLX multi-vendor PRIME GPU offload
On 02/08/2019 11:19 AM, Andy Ritger wrote:
(I'll omit EGL and Vulkan for the moment, for the sake of focus, and those
APIs have programmatic ways to enumerate and select GPUs. Though, some
of what we decide here for GLX we may want to leverage for other APIs.)
Today, GLX implementations loaded into the X server register themselves
on a per-screen basis, GLXVND in the server dispatches GLX requests to
the registered vendor per screen, and libglvnd determines the client-side
vendor library to use by querying the per-screen GLX_VENDOR_NAMES_EXT
string from the X server (e.g., "mesa" or "nvidia").
The GLX_VENDOR_NAMES_EXT string can be overridden within libglvnd
through the __GLX_VENDOR_LIBRARY_NAME environment variable, though I
don't believe that is used much currently.
To enable GLX to be used in a multi-vendor PRIME GPU offload environment,
it seems there are several desirable user-visible behaviors:
* By default, users should get the same behavior we have today (i.e.,
the GLX implementation used within the client and the server, for an X
screen, is dictated by the X driver of the X screen).
* The user should be able to request a different GLX vendor for use on a
per-process basis through either an environment variable (potentially
reusing __GLX_VENDOR_LIBRARY_NAME) or possibly a future application
profile mechanism in libglvnd.
* To make configuration optionally more "portable", the selection override
mechanism should be able to refer to more generic names like
"performance" or "battery", and those generic names should be mapped
to specific GPUs/vendors on a per-system basis.
* To make configuration optionally more explicit, the selection override
mechanism should be able to distinguish between individual GPUs by
using hardware specific identifiers such as PCI BusID-based names like
what DRI_PRIME currently honors (e.g., "pci-_03_00_0").
Do those behaviors seem reasonable?
If so, it seems like there are two general directions we could take to
implement that infrastructure in client-side libglvnd and GLXVND within
the X server, if the user or application profile requests a particular
vendor, either by vendor name (e.g., "mesa"/"nvidia"), functional
name (e.g., "battery"/"performance"), or hardware-based name (e.g.,
"pci-_03_00_0"/pci-_01_00_0"):
(1) If configured for PRIME GPU offloading (environment variable or
application profile), client-side libglvnd could load the possible
libGLX_${vendor}.so libraries it finds, and call into each to
find which vendor (and possibly which GPU) matches the specified
string. Once a vendor is selected, the vendor library could optionally
tell the X server which GLX vendor to use server-side for this
client connection.
(2) The GLX implementations within the X server could, when registering
with GLXVND, tell GLXVND which screens they can support for PRIME
GPU offloading. That list could be queried by client-side libglvnd,
and then used to interpret __GLX_VENDOR_LIBRARY_NAME and pick the
corresponding vendor library to load. Client-side would tell the X
server which GLX vendor to use server-side for this client connection.
In either direction, if the user-requested string is a hardware-based
name ("pci-_03_00_0"), the GLX vendor library presumably needs to be
told that GPU, so that the vendor implementation can use the right GPU
(in the case that the vendor supports multiple GPUs in the system).
But, both (1) and (2) are really just points on a continuum. I suppose
the more general question is: how much of the implementation should go
in the server and how much should go in the client?
At one extreme, the client could do nearly all the work (with the
practical downside of potentially loading multiple vendor libraries in
order to interpret __GLX_VENDOR_LIBRARY_NAME).
At the other extreme, the server could do nearly all the work of
generating the possible __GLX_VENDOR_LIBRARY_NAME strings (with the
practical downside of each server-side GLX vendor needing to enumerate
the GPUs it can drive, in order to generate the hardware-specific
identifiers).
I'm not sure where on that spectrum it makes the most sense to land,
and I'm curious what others think.
Thanks,
- Andy
For a more concrete example, this is what I've been working on for a
client-based interface:
https://github.com/kbrenneman/libglvnd/tree/libglx-gpu-offloading
For this design, I've tried to keep the interface as simple as possible
and to impose as few requirements or assumptions as possible. The basic
idea behind it is that the only thing that a GLX application has to care
about is calling GLX functions, and the only thing that libglvnd has to
care about is forwarding those functions to the correct vendor library.
The general design is this:
* Libglvnd gets a list of alternate vendor libraries from an app profile
(config file, environment variable, whatever)
* For each vendor in
