Message-ID: <CAA+WOBtmbPHigscFQCFgDo=9WSM6V-JMXGCO7orP=01XOqTPHQ@mail.gmail.com>
Date: Tue, 4 Nov 2025 01:53:33 +0200
From: Mohamed Ahmed <mohamedahmedegypt2001@...il.com>
To: James Jones <jajones@...dia.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	Mary Guillemard <mary@...y.zone>, Faith Ekstrand <faith.ekstrand@...labora.com>, 
	Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	nouveau@...ts.freedesktop.org
Subject: Re: [PATCH v4 2/5] drm/nouveau/uvmm: Allow larger pages

Thanks a lot for the shout out! Looking more at things, the logic here
is actually redundant. It was originally copied over directly from the
bo allocation code to stay on the safer side (basically the idea back
then was to make both the bo and vmm sides match exactly). We aren't
at risk of having an aligned address that is in the wrong memory type
because the bo allocation code (nouveau_bo.c:321) forces anything that
has the GART flag to have a page size of 4K. Anything getting a page
size higher than that is exclusively VRAM only. Additionally,
currently things marked VRAM only don't get evicted to host memory
except under high memory pressure and in that case, the context is
paused until the objects in question are paged back in, so we also
don't have to worry about memory placement there.

The memory placement check in the vmm code could be removed but I am
leaning more towards leaving it as is just to stay on the safer side.
At the same time, it would be more useful to keep it for the future as
one of the future investigation targets that we want to look into is
all the memory placement rules because the "only 4K is allowed for
host memory" limit that nouveau imposes is a source of many pains in
userspace (originally thought to be a HW thing but seems it's actually
not), and having the checks on both bo and vmm paths would help
starting out with that.

Thanks a lot again,
Mohamed

On Fri, Oct 31, 2025 at 7:01 PM James Jones <jajones@...dia.com> wrote:
>
> On 10/31/25 03:49, Mohamed Ahmed wrote:
> > From: Mary Guillemard <mary@...y.zone>
> >
> > Now that everything in UVMM knows about the variable page shift, we can
> > select larger values.
> >
> > The proposed approach relies on nouveau_bo::page unless if it would cause
> > alignment issues (in which case we fall back to searching for an
> > appropriate shift)
> >
> > Signed-off-by: Mary Guillemard <mary@...y.zone>
> > Co-developed-by: Mohamed Ahmed <mohamedahmedegypt2001@...il.com>
> > Signed-off-by: Mohamed Ahmed <mohamedahmedegypt2001@...il.com>
> > ---
> >   drivers/gpu/drm/nouveau/nouveau_uvmm.c | 60 +++++++++++++++++++++++++-
> >   1 file changed, 58 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_uvmm.c b/drivers/gpu/drm/nouveau/nouveau_uvmm.c
> > index 2cd0835b05e8..ab8933b88337 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_uvmm.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_uvmm.c
> > @@ -454,6 +454,62 @@ op_unmap_prepare_unwind(struct drm_gpuva *va)
> >       drm_gpuva_insert(va->vm, va);
> >   }
> >
> > +static bool
> > +op_map_aligned_to_page_shift(const struct drm_gpuva_op_map *op, u8 page_shift)
> > +{
> > +     u64 non_page_bits = (1ULL << page_shift) - 1;
> > +
> > +     return (op->va.addr & non_page_bits) == 0 &&
> > +            (op->va.range & non_page_bits) == 0 &&
> > +            (op->gem.offset & non_page_bits) == 0;
> > +}
> > +
> > +static u8
> > +select_page_shift(struct nouveau_uvmm *uvmm, struct drm_gpuva_op_map *op)
> > +{
> > +     struct nouveau_bo *nvbo = nouveau_gem_object(op->gem.obj);
> > +
> > +     /* nouveau_bo_fixup_align() guarantees that the page size will be aligned
> > +      * for most cases, but it can't handle cases where userspace allocates with
> > +      * a size and then binds with a smaller granularity. So in order to avoid
> > +      * breaking old userspace, we need to ensure that the VA is actually
> > +      * aligned before using it, and if it isn't, then we downgrade to the first
> > +      * granularity that will fit, which is optimal from a correctness and
> > +      * performance perspective.
> > +      */
> > +     if (op_map_aligned_to_page_shift(op, nvbo->page))
> > +             return nvbo->page;
> > +
> > +     struct nouveau_mem *mem = nouveau_mem(nvbo->bo.resource);
> > +     struct nvif_vmm *vmm = &uvmm->vmm.vmm;
> > +     int i;
> > +
> > +     /* If the given granularity doesn't fit, let's find one that will fit. */
> > +     for (i = 0; i < vmm->page_nr; i++) {
> > +             /* Ignore anything that is bigger or identical to the BO preference. */
> > +             if (vmm->page[i].shift >= nvbo->page)
> > +                     continue;
> > +
> > +             /* Skip incompatible domains. */
> > +             if ((mem->mem.type & NVIF_MEM_VRAM) && !vmm->page[i].vram)
> > +                     continue;
> > +             if ((mem->mem.type & NVIF_MEM_HOST) &&
> > +                 (!vmm->page[i].host || vmm->page[i].shift > PAGE_SHIFT))
> > +                     continue;
>
> This logic doesn't seem correct. I'm not sure why there's a need to
> limit the page size on the host memory type, but assuming there is due
> to nouveau architecture or HW limitations I'm not aware of, it should be
> applied universally, not just when falling back due to misaligned
> addresses. You can get lucky and have aligned addresses regardless of
> the target page size. Hence, this check would need to precede the above
> early-out for the case where op_map_aligned_to_page_shift() succeeds.
>
> Thanks,
> -James
>
> > +             /* If it fits, return the proposed shift. */
> > +             if (op_map_aligned_to_page_shift(op, vmm->page[i].shift))
> > +                     return vmm->page[i].shift;
> > +     }
> > +
> > +     /* If we get here then nothing can reconcile the requirements. This should never
> > +      * happen.
> > +      */
> > +     WARN_ON(1);
> > +
> > +     return PAGE_SHIFT;
> > +}
> > +
> >   static void
> >   nouveau_uvmm_sm_prepare_unwind(struct nouveau_uvmm *uvmm,
> >                              struct nouveau_uvma_prealloc *new,
> > @@ -506,7 +562,7 @@ nouveau_uvmm_sm_prepare_unwind(struct nouveau_uvmm *uvmm,
> >                       if (vmm_get_range)
> >                               nouveau_uvmm_vmm_put(uvmm, vmm_get_start,
> >                                                    vmm_get_range,
> > -                                                  PAGE_SHIFT);
> > +                                                  select_page_shift(uvmm, &op->map));
> >                       break;
> >               }
> >               case DRM_GPUVA_OP_REMAP: {
> > @@ -599,7 +655,7 @@ op_map_prepare(struct nouveau_uvmm *uvmm,
> >
> >       uvma->region = args->region;
> >       uvma->kind = args->kind;
> > -     uvma->page_shift = PAGE_SHIFT;
> > +     uvma->page_shift = select_page_shift(uvmm, op);
> >
> >       drm_gpuva_map(&uvmm->base, &uvma->va, op);
> >
>

Powered by blists - more mailing lists