[PATCH v7 15/23] nova-core: mm: Add multi-page mapping API to VMM

[Joel Fernandes <joelagnelf@nvidia.com>]

Add the page table mapping and unmapping API to the Virtual Memory
Manager, implementing a two-phase prepare/execute model suitable for
use both inside and outside the DMA fence signalling critical path.

Cc: Nikola Djukic <ndjukic@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 drivers/gpu/nova-core/mm/vmm.rs | 347 +++++++++++++++++++++++++++++++-
 1 file changed, 345 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/nova-core/mm/vmm.rs b/drivers/gpu/nova-core/mm/vmm.rs
index 9e57916017ed..af3daccbf958 100644
--- a/drivers/gpu/nova-core/mm/vmm.rs
+++ b/drivers/gpu/nova-core/mm/vmm.rs
@@ -17,15 +17,25 @@
         GpuBuddyParams, //
     },
     prelude::*,
+    rbtree::{RBTree, RBTreeNode},
     sizes::SZ_4K, //
 };
 
+use core::cell::Cell;
 use core::ops::Range;
 
 use crate::mm::{
     pagetable::{
-        walk::{PtWalk, WalkResult},
-        MmuVersion, //
+        walk::{
+            PtWalk,
+            WalkPdeResult,
+            WalkResult, //
+        },
+        DualPde,
+        MmuVersion,
+        PageTableLevel,
+        Pde,
+        Pte, //
     },
     GpuMm,
     Pfn,
@@ -46,6 +56,74 @@ pub(crate) struct Vmm {
     page_table_allocs: KVec<Pin<KBox<AllocatedBlocks>>>,
     /// Buddy allocator for virtual address range tracking.
     virt_buddy: GpuBuddy,
+    /// Prepared PT pages pending PDE installation, keyed by `install_addr`.
+    ///
+    /// Populated by `Vmm` mapping prepare phase and drained in the execute phase.
+    /// Shared by all pending maps in the `Vmm`, thus preventing races where 2
+    /// maps might be trying to install the same page table/directory entry pointer.
+    pt_pages: RBTree<VramAddress, PreparedPtPage>,
+}
+
+/// A pre-allocated and zeroed page table page.
+///
+/// Created during the mapping prepare phase and consumed during the mapping execute phase.
+/// Stored in an [`RBTree`] keyed by the PDE slot address (`install_addr`).
+struct PreparedPtPage {
+    /// The allocated and zeroed page table page.
+    alloc: Pin<KBox<AllocatedBlocks>>,
+    /// Page table level -- needed to determine if this PT page is for a dual PDE.
+    level: PageTableLevel,
+}
+
+/// Multi-page prepared mapping -- VA range allocated, ready for execute.
+///
+/// Produced by [`Vmm::prepare_map()`], consumed by [`Vmm::execute_map()`].
+/// The struct owns the VA space allocation between prepare and execute phases.
+pub(crate) struct PreparedMapping {
+    vfn_start: Vfn,
+    num_pages: usize,
+    vfn_alloc: Pin<KBox<AllocatedBlocks>>,
+}
+
+/// Result of a mapping operation -- tracks the active mapped range.
+///
+/// Returned by [`Vmm::execute_map()`] and [`Vmm::map_pages()`].
+/// Owns the VA allocation; the VA range is freed when this is dropped.
+/// Callers must call [`Vmm::unmap_pages()`] before dropping to invalidate
+/// PTEs (dropping only frees the VA range, not the PTE entries).
+pub(crate) struct MappedRange {
+    pub(crate) vfn_start: Vfn,
+    pub(crate) num_pages: usize,
+    /// VA allocation -- freed when [`MappedRange`] is dropped.
+    _vfn_alloc: Pin<KBox<AllocatedBlocks>>,
+    /// Logs a warning if dropped without unmapping.
+    _drop_guard: MustUnmapGuard,
+}
+
+/// Guard that logs a warning once if a [`MappedRange`] is dropped without
+/// calling [`Vmm::unmap_pages()`].
+struct MustUnmapGuard {
+    armed: Cell<bool>,
+}
+
+impl MustUnmapGuard {
+    const fn new() -> Self {
+        Self {
+            armed: Cell::new(true),
+        }
+    }
+
+    fn disarm(&self) {
+        self.armed.set(false);
+    }
+}
+
+impl Drop for MustUnmapGuard {
+    fn drop(&mut self) {
+        if self.armed.get() {
+            kernel::pr_warn_once!("MappedRange dropped without calling unmap_pages()\n");
+        }
+    }
 }
 
 impl Vmm {
@@ -72,6 +150,7 @@ pub(crate) fn new(
             mmu_version,
             page_table_allocs: KVec::new(),
             virt_buddy,
+            pt_pages: RBTree::new(),
         })
     }
 
@@ -127,4 +206,268 @@ pub(crate) fn read_mapping(&self, mm: &GpuMm, vfn: Vfn) -> Result<Option<Pfn>> {
             WalkResult::Unmapped { .. } | WalkResult::PageTableMissing => Ok(None),
         }
     }
+
+    /// Allocate and zero a physical page table page for a specific PDE slot.
+    /// Called during the map prepare phase.
+    fn alloc_and_zero_page_table(
+        &mut self,
+        mm: &GpuMm,
+        level: PageTableLevel,
+    ) -> Result<PreparedPtPage> {
+        let params = GpuBuddyAllocParams {
+            start_range_address: 0,
+            end_range_address: 0,
+            size_bytes: SZ_4K as u64,
+            min_block_size_bytes: SZ_4K as u64,
+            buddy_flags: BuddyFlags::try_new(0)?,
+        };
+        let blocks = KBox::pin_init(mm.buddy().alloc_blocks(&params), GFP_KERNEL)?;
+
+        // Get page's VRAM address from the allocation.
+        let page_vram = VramAddress::new(blocks.iter().next().ok_or(ENOMEM)?.offset());
+
+        // Zero via PRAMIN.
+        let mut window = mm.pramin().window()?;
+        let base = page_vram.raw();
+        for off in (0..PAGE_SIZE).step_by(8) {
+            window.try_write64(base + off, 0)?;
+        }
+
+        Ok(PreparedPtPage {
+            alloc: blocks,
+            level,
+        })
+    }
+
+    /// Ensure all intermediate page table pages are prepared for a [`Vfn`]. Just
+    /// finds out which PDE pages are missing, allocates pages for them, and defers
+    /// installation to the execute phase.
+    ///
+    /// PRAMIN is released before each allocation and re-acquired after. Memory
+    /// allocations outside of holding this lock to prevent deadlocks with fence signalling
+    /// critical path.
+    fn ensure_pte_path(&mut self, mm: &GpuMm, vfn: Vfn) -> Result {
+        let walker = PtWalk::new(self.pdb_addr, self.mmu_version);
+        let max_iter = 2 * self.mmu_version.pde_level_count();
+
+        // Keep looping until all PDE levels are resolved.
+        for _ in 0..max_iter {
+            let mut window = mm.pramin().window()?;
+
+            // Walk PDE levels. The closure checks self.pt_pages for prepared-but-uninstalled
+            // pages, letting the walker continue through them as if they were installed in HW.
+            // The walker keeps calling the closure to get these "prepared but not installed" pages.
+            let result = walker.walk_pde_levels(&mut window, vfn, |install_addr| {
+                self.pt_pages
+                    .get(&install_addr)
+                    .and_then(|p| Some(VramAddress::new(p.alloc.iter().next()?.offset())))
+            })?;
+
+            match result {
+                WalkPdeResult::Complete { .. } => {
+                    // All PDE levels resolved.
+                    return Ok(());
+                }
+                WalkPdeResult::Missing {
+                    install_addr,
+                    level,
+                } => {
+                    // Drop PRAMIN before allocation.
+                    drop(window);
+                    let page = self.alloc_and_zero_page_table(mm, level)?;
+                    let node = RBTreeNode::new(install_addr, page, GFP_KERNEL)?;
+                    let old = self.pt_pages.insert(node);
+                    if old.is_some() {
+                        kernel::pr_warn_once!(
+                            "VMM: duplicate install_addr in pt_pages (internal consistency error)\n"
+                        );
+                        return Err(EIO);
+                    }
+                    // Loop: re-acquire PRAMIN and re-walk from root.
+                }
+            }
+        }
+
+        Err(EIO)
+    }
+
+    /// Prepare resources for mapping `num_pages` pages.
+    ///
+    /// Allocates a contiguous VA range, then walks the hierarchy per-VFN to prepare pages
+    /// for all missing PDEs. Returns a [`PreparedMapping`] with the VA allocation.
+    ///
+    /// If `va_range` is not `None`, the VA range is constrained to the given range. Safe
+    /// to call outside the fence signalling critical path.
+    pub(crate) fn prepare_map(
+        &mut self,
+        mm: &GpuMm,
+        num_pages: usize,
+        va_range: Option<Range<u64>>,
+    ) -> Result<PreparedMapping> {
+        if num_pages == 0 {
+            return Err(EINVAL);
+        }
+
+        // Pre-reserve so execute_map() can use push_within_capacity (no alloc in
+        // fence signalling critical path).
+        // Upper bound on page table pages needed for the full tree (PTE pages + PDE
+        // pages at all levels).
+        let pt_upper_bound = self.mmu_version.pt_pages_upper_bound(num_pages);
+        self.page_table_allocs.reserve(pt_upper_bound, GFP_KERNEL)?;
+
+        // Allocate contiguous VA range.
+        let (vfn_start, vfn_alloc) = self.alloc_vfn_range(num_pages, va_range)?;
+
+        // Walk the hierarchy per-VFN to prepare pages for all missing PDEs.
+        for i in 0..num_pages {
+            let vfn = Vfn::new(vfn_start.raw() + i as u64);
+            self.ensure_pte_path(mm, vfn)?;
+        }
+
+        Ok(PreparedMapping {
+            vfn_start,
+            num_pages,
+            vfn_alloc,
+        })
+    }
+
+    /// Execute a prepared multi-page mapping.
+    ///
+    /// Drain prepared PT pages and install PDEs followed by single TLB flush.
+    pub(crate) fn execute_map(
+        &mut self,
+        mm: &GpuMm,
+        prepared: PreparedMapping,
+        pfns: &[Pfn],
+        writable: bool,
+    ) -> Result<MappedRange> {
+        if pfns.len() != prepared.num_pages {
+            return Err(EINVAL);
+        }
+
+        let PreparedMapping {
+            vfn_start,
+            num_pages,
+            vfn_alloc,
+        } = prepared;
+
+        let walker = PtWalk::new(self.pdb_addr, self.mmu_version);
+        let mut window = mm.pramin().window()?;
+
+        // First, drain self.pt_pages, install all pending PDEs.
+        let mut cursor = self.pt_pages.cursor_front_mut();
+        while let Some(c) = cursor {
+            let (next, node) = c.remove_current();
+            let (install_addr, page) = node.to_key_value();
+            let page_vram = VramAddress::new(page.alloc.iter().next().ok_or(ENOMEM)?.offset());
+
+            if page.level == self.mmu_version.dual_pde_level() {
+                let new_dpde = DualPde::new_small(self.mmu_version, Pfn::from(page_vram));
+                new_dpde.write(&mut window, install_addr)?;
+            } else {
+                let new_pde = Pde::new_vram(self.mmu_version, Pfn::from(page_vram));
+                new_pde.write(&mut window, install_addr)?;
+            }
+
+            // Track the allocated pages in the `Vmm`.
+            self.page_table_allocs
+                .push_within_capacity(page.alloc)
+                .map_err(|_| ENOMEM)?;
+
+            cursor = next;
+        }
+
+        // Next, write PTEs (all PDEs now installed in HW).
+        for (i, &pfn) in pfns.iter().enumerate() {
+            let vfn = Vfn::new(vfn_start.raw() + i as u64);
+            let result = walker.walk_to_pte_lookup_with_window(&mut window, vfn)?;
+
+            match result {
+                WalkResult::Unmapped { pte_addr } | WalkResult::Mapped { pte_addr, .. } => {
+                    let pte = Pte::new_vram(self.mmu_version, pfn, writable);
+                    pte.write(&mut window, pte_addr)?;
+                }
+                WalkResult::PageTableMissing => {
+                    kernel::pr_warn_once!("VMM: page table missing for VFN {vfn:?}\n");
+                    return Err(EIO);
+                }
+            }
+        }
+
+        drop(window);
+
+        // Finally, flush the TLB.
+        mm.tlb().flush(self.pdb_addr)?;
+
+        Ok(MappedRange {
+            vfn_start,
+            num_pages,
+            _vfn_alloc: vfn_alloc,
+            _drop_guard: MustUnmapGuard::new(),
+        })
+    }
+
+    /// Map pages doing prepare and execute in the same call.
+    ///
+    /// This is a convenience wrapper for callers outside the fence signalling critical
+    /// path (e.g., BAR mappings). For DRM usecases, [`Vmm::prepare_map()`] and
+    /// [`Vmm::execute_map()`] will be called separately.
+    pub(crate) fn map_pages(
+        &mut self,
+        mm: &GpuMm,
+        pfns: &[Pfn],
+        va_range: Option<Range<u64>>,
+        writable: bool,
+    ) -> Result<MappedRange> {
+        if pfns.is_empty() {
+            return Err(EINVAL);
+        }
+
+        // Check if provided VA range is sufficient (if provided).
+        if let Some(ref range) = va_range {
+            let required = pfns.len().checked_mul(PAGE_SIZE).ok_or(EOVERFLOW)? as u64;
+            let available = range.end.checked_sub(range.start).ok_or(EINVAL)?;
+            if available < required {
+                return Err(EINVAL);
+            }
+        }
+
+        let prepared = self.prepare_map(mm, pfns.len(), va_range)?;
+        self.execute_map(mm, prepared, pfns, writable)
+    }
+
+    /// Unmap all pages in a [`MappedRange`] with a single TLB flush.
+    ///
+    /// Takes the range by value (consumes it), then invalidates PTEs for the range,
+    /// flushes the TLB, then drops the range (freeing the VA). PRAMIN lock is held.
+    pub(crate) fn unmap_pages(&mut self, mm: &GpuMm, range: MappedRange) -> Result {
+        let walker = PtWalk::new(self.pdb_addr, self.mmu_version);
+        let invalid_pte = Pte::invalid(self.mmu_version);
+
+        let mut window = mm.pramin().window()?;
+        for i in 0..range.num_pages {
+            let vfn = Vfn::new(range.vfn_start.raw() + i as u64);
+            let result = walker.walk_to_pte_lookup_with_window(&mut window, vfn)?;
+
+            match result {
+                WalkResult::Mapped { pte_addr, .. } | WalkResult::Unmapped { pte_addr } => {
+                    invalid_pte.write(&mut window, pte_addr)?;
+                }
+                WalkResult::PageTableMissing => {
+                    continue;
+                }
+            }
+        }
+        drop(window);
+
+        mm.tlb().flush(self.pdb_addr)?;
+
+        // TODO: Internal page table pages (PDE, PTE pages) are still kept around.
+        // This is by design as repeated maps/unmaps will be fast. As a future TODO,
+        // we can add a reclaimer here to reclaim if VRAM is short. For now, the PT
+        // pages are dropped once the `Vmm` is dropped.
+
+        range._drop_guard.disarm(); // Unmap complete, Ok to drop MappedRange.
+        Ok(())
+    }
 }
-- 
2.34.1