Re: [PATCH v5 2/2] drm/xe: Enable 2M pages in xe_migrate_vram

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Oct 13, 2025 at 11:22:10AM -0600, Summers, Stuart wrote:
> On Mon, 2025-10-13 at 10:14 -0700, Matthew Brost wrote:
> > On Mon, Oct 13, 2025 at 11:08:03AM -0600, Summers, Stuart wrote:
> > > On Sun, 2025-10-12 at 20:45 -0700, Matthew Brost wrote:
> > > > Using 2M pages in xe_migrate_vram has two benefits: we issue
> > > > fewer
> > > > instructions per 2M copy (1 vs. 512), and the cache hit rate
> > > > should
> > > > be
> > > > higher. This results in increased copy engine bandwidth, as shown
> > > > by
> > > > benchmark IGTs.
> > > > 
> > > > Enable 2M pages by reserving PDEs in the migrate VM and using 2M
> > > > pages
> > > > in xe_migrate_vram if the DMA address order matches 2M.
> > > > 
> > > > v2:
> > > >  - Reuse build_pt_update_batch_sram (Stuart)
> > > >  - Fix build_pt_update_batch_sram for PAGE_SIZE > 4K
> > > > v3:
> > > >  - More fixes for PAGE_SIZE > 4K, align chunk, decrement chunk as
> > > > needed
> > > >  - Use stack incr var in xe_migrate_vram_use_pde (Stuart)
> > > > v4:
> > > >  - Split PAGE_SIZE > 4K fix out in different patch (Stuart)
> > > > 
> > > > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > > > ---
> > > >  drivers/gpu/drm/xe/xe_migrate.c | 53
> > > > ++++++++++++++++++++++++++++---
> > > > --
> > > >  1 file changed, 45 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/xe/xe_migrate.c
> > > > b/drivers/gpu/drm/xe/xe_migrate.c
> > > > index 216fc0ec2bb7..4ca48dd1cfd8 100644
> > > > --- a/drivers/gpu/drm/xe/xe_migrate.c
> > > > +++ b/drivers/gpu/drm/xe/xe_migrate.c
> > > > @@ -57,6 +57,13 @@ struct xe_migrate {
> > > >         u64 usm_batch_base_ofs;
> > > >         /** @cleared_mem_ofs: VM offset of @cleared_bo. */
> > > >         u64 cleared_mem_ofs;
> > > > +       /** @large_page_copy_ofs: VM offset of 2M pages used for
> > > > large copies */
> > > > +       u64 large_page_copy_ofs;
> > > > +       /**
> > > > +        * @large_page_copy_pdes: BO offset to writeout 2M pages
> > > > (PDEs) used for
> > > > +        * large copies
> > > > +        */
> > > > +       u64 large_page_copy_pdes;
> > > >         /**
> > > >          * @fence: dma-fence representing the last migration job
> > > > batch.
> > > >          * Protected by @job_mutex.
> > > > @@ -288,6 +295,12 @@ static int xe_migrate_prepare_vm(struct
> > > > xe_tile
> > > > *tile, struct xe_migrate *m,
> > > >                           (i + 1) * 8, u64, entry);
> > > >         }
> > > >  
> > > > +       /* Reserve 2M PDEs */
> > > > +       level = 1;
> > > > +       m->large_page_copy_ofs = NUM_PT_SLOTS <<
> > > > xe_pt_shift(level);
> > > > +       m->large_page_copy_pdes = map_ofs + XE_PAGE_SIZE * level
> > > > +
> > > > +               NUM_PT_SLOTS * 8;
> > > > +
> > > >         /* Set up a 1GiB NULL mapping at 255GiB offset. */
> > > >         level = 2;
> > > >         xe_map_wr(xe, &bo->vmap, map_ofs + XE_PAGE_SIZE * level +
> > > > 255
> > > > * 8, u64,
> > > > @@ -1778,10 +1791,10 @@ static u32 pte_update_cmd_size(u64 size)
> > > >  static void build_pt_update_batch_sram(struct xe_migrate *m,
> > > >                                        struct xe_bb *bb, u32
> > > > pt_offset,
> > > >                                        struct drm_pagemap_addr
> > > > *sram_addr,
> > > > -                                      u32 size)
> > > > +                                      u32 size, int level)
> > > >  {
> > > >         u16 pat_index = tile_to_xe(m->tile)-
> > > > >pat.idx[XE_CACHE_WB];
> > > > -       u64 gpu_page_size = 0x1ull << xe_pt_shift(0);
> > > > +       u64 gpu_page_size = 0x1ull << xe_pt_shift(level);
> > > >         u32 ptes;
> > > >         int i = 0;
> > > >  
> > > > @@ -1808,7 +1821,7 @@ static void
> > > > build_pt_update_batch_sram(struct
> > > > xe_migrate *m,
> > > >  again:
> > > >                         pte = m->q->vm->pt_ops-
> > > > >pte_encode_addr(m-
> > > > > tile->xe,
> > > >                                                                 a
> > > > ddr,
> > > > pat_index,
> > > > -
> > > >                                                                0,
> > > > false, 0);
> > > > +                                                               l
> > > > evel
> > > > , false, 0);
> > > >                         bb->cs[bb->len++] = lower_32_bits(pte);
> > > >                         bb->cs[bb->len++] = upper_32_bits(pte);
> > > >  
> > > > @@ -1826,6 +1839,19 @@ static void
> > > > build_pt_update_batch_sram(struct
> > > > xe_migrate *m,
> > > >         }
> > > >  }
> > > >  
> > > > +static bool xe_migrate_vram_use_pde(struct drm_pagemap_addr
> > > > *sram_addr,
> > > > +                                   unsigned long size)
> > > > +{
> > > > +       u32 large_size = (0x1 << xe_pt_shift(1));
> > > > +       unsigned long i, incr = large_size / PAGE_SIZE;
> > > > +
> > > > +       for (i = 0; i < DIV_ROUND_UP(size, PAGE_SIZE); i += incr)
> > > > +               if (PAGE_SIZE << sram_addr[i].order !=
> > > > large_size)
> > > > +                       return false;
> > > > +
> > > > +       return true;
> > > > +}
> > > > +
> > > >  enum xe_migrate_copy_dir {
> > > >         XE_MIGRATE_COPY_TO_VRAM,
> > > >         XE_MIGRATE_COPY_TO_SRAM,
> > > > @@ -1855,6 +1881,7 @@ static struct dma_fence
> > > > *xe_migrate_vram(struct
> > > > xe_migrate *m,
> > > >                 PAGE_SIZE : 4;
> > > >         int err;
> > > >         unsigned long i, j;
> > > > +       bool use_pde = xe_migrate_vram_use_pde(sram_addr, len +
> > > > sram_offset);
> > > >  
> > > >         if (drm_WARN_ON(&xe->drm, (len & XE_CACHELINE_MASK) ||
> > > >                         (sram_offset | vram_addr) &
> > > > XE_CACHELINE_MASK))
> > > > @@ -1879,7 +1906,7 @@ static struct dma_fence
> > > > *xe_migrate_vram(struct
> > > > xe_migrate *m,
> > > >          * struct drm_pagemap_addr. Ensure this is the case even
> > > > with
> > > > higher
> > > >          * orders.
> > > >          */
> > > > -       for (i = 0; i < npages;) {
> > > > +       for (i = 0; !use_pde && i < npages;) {
> > > 
> > > What if the CPU page size is larger than 2M? Don't we still want
> > > this?
> > > 
> > 
> > I'm not handling this but I believe CPU pages are at most 64k on ARM,
> > power, or longsoon. I could add an assert I suppose to make sure this
> > unhandled case never occurs.
> 
> So according to https://docs.kernel.org/admin-guide/mm/hugetlbpage.html
> we can potentially get 4M-256M pages on some architectures. Maybe we
> want to say we aren't supporting these? In which case, yeah I think
> having an assertion here would be helpful. Would it be easier just to
> apply this same code if sram_addr[i].order > != 2M?
> 

I only enable the PDE path if order == 2M. See what xe_migrate_vram_use_pde does.

Matt

> Thanks,
> Stuart
> 
> > 
> > Matt
> > 
> > > Thanks,
> > > Stuart
> > > 
> > > >                 unsigned int order = sram_addr[i].order;
> > > >  
> > > >                 for (j = 1; j < NR_PAGES(order) && i + j <
> > > > npages;
> > > > j++)
> > > > @@ -1889,16 +1916,26 @@ static struct dma_fence
> > > > *xe_migrate_vram(struct xe_migrate *m,
> > > >                 i += NR_PAGES(order);
> > > >         }
> > > >  
> > > > -       build_pt_update_batch_sram(m, bb, pt_slot * XE_PAGE_SIZE,
> > > > -                                  sram_addr, len + sram_offset);
> > > > +       if (use_pde)
> > > > +               build_pt_update_batch_sram(m, bb, m-
> > > > > large_page_copy_pdes,
> > > > +                                          sram_addr, len +
> > > > sram_offset, 1);
> > > > +       else
> > > > +               build_pt_update_batch_sram(m, bb, pt_slot *
> > > > XE_PAGE_SIZE,
> > > > +                                          sram_addr, len +
> > > > sram_offset, 0);
> > > >  
> > > >         if (dir == XE_MIGRATE_COPY_TO_VRAM) {
> > > > -               src_L0_ofs = xe_migrate_vm_addr(pt_slot, 0) +
> > > > sram_offset;
> > > > +               if (use_pde)
> > > > +                       src_L0_ofs = m->large_page_copy_ofs +
> > > > sram_offset;
> > > > +               else
> > > > +                       src_L0_ofs = xe_migrate_vm_addr(pt_slot,
> > > > 0) +
> > > > sram_offset;
> > > >                 dst_L0_ofs = xe_migrate_vram_ofs(xe, vram_addr,
> > > > false);
> > > >  
> > > >         } else {
> > > >                 src_L0_ofs = xe_migrate_vram_ofs(xe, vram_addr,
> > > > false);
> > > > -               dst_L0_ofs = xe_migrate_vm_addr(pt_slot, 0) +
> > > > sram_offset;
> > > > +               if (use_pde)
> > > > +                       dst_L0_ofs = m->large_page_copy_ofs +
> > > > sram_offset;
> > > > +               else
> > > > +                       dst_L0_ofs = xe_migrate_vm_addr(pt_slot,
> > > > 0) +
> > > > sram_offset;
> > > >         }
> > > >  
> > > >         bb->cs[bb->len++] = MI_BATCH_BUFFER_END;
> > > 
>