Re: [PATCH v10 07/21] gpu: nova-core: mm: Add TLB flush support

Joel Fernandes
2026-04-07T07:44:55Z

On 4/2/2026 1:59 AM, Matthew Brost wrote:
> On Tue, Mar 31, 2026 at 05:20:34PM -0400, Joel Fernandes wrote:
>> Add TLB (Translation Lookaside Buffer) flush support for GPU MMU.
>>
>> After modifying page table entries, the GPU's TLB must be invalidated
>> to ensure the new mappings take effect. The Tlb struct provides flush
>> functionality through BAR0 registers.
>>
>> The flush operation writes the page directory base address and triggers
>> an invalidation, polling for completion with a 2 second timeout matching
>> the Nouveau driver.
>>
>> Cc: Nikola Djukic <ndjukic@nvidia.com>
>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>> ---
>> drivers/gpu/nova-core/mm.rs | 1 +
>> drivers/gpu/nova-core/mm/tlb.rs | 95 +++++++++++++++++++++++++++++++++
>> drivers/gpu/nova-core/regs.rs | 42 +++++++++++++++
>> 3 files changed, 138 insertions(+)
>> create mode 100644 drivers/gpu/nova-core/mm/tlb.rs
>>
>> diff --git a/drivers/gpu/nova-core/mm.rs b/drivers/gpu/nova-core/mm.rs
>> index 8f3089a5fa88..cfe9cbe11d57 100644
>> --- a/drivers/gpu/nova-core/mm.rs
>> +++ b/drivers/gpu/nova-core/mm.rs
>> @@ -5,6 +5,7 @@
>> #![expect(dead_code)]
>>
>> pub(crate) mod pramin;
>> +pub(crate) mod tlb;
>>
>> use kernel::sizes::SZ_4K;
>>
>> diff --git a/drivers/gpu/nova-core/mm/tlb.rs b/drivers/gpu/nova-core/mm/tlb.rs
>> new file mode 100644
>> index 000000000000..cd3cbcf4c739
>> --- /dev/null
>> +++ b/drivers/gpu/nova-core/mm/tlb.rs
>> @@ -0,0 +1,95 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +//! TLB (Translation Lookaside Buffer) flush support for GPU MMU.
>> +//!
>> +//! After modifying page table entries, the GPU's TLB must be flushed to
>> +//! ensure the new mappings take effect. This module provides TLB flush
>> +//! functionality for virtual memory managers.
>> +//!
>> +//! # Example
>> +//!
>> +//! ```ignore
>> +//! use crate::mm::tlb::Tlb;
>> +//!
>> +//! fn page_table_update(tlb: &Tlb, pdb_addr: VramAddress) -> Result<()> {
>> +//! // ... modify page tables ...
>> +//!
>> +//! // Flush TLB to make changes visible (polls for completion).
>> +//! tlb.flush(pdb_addr)?;
>> +//!
>> +//! Ok(())
>> +//! }
>> +//! ```
>> +
>> +use kernel::{
>> + devres::Devres,
>> + io::poll::read_poll_timeout,
>> + io::Io,
>> + new_mutex,
>> + prelude::*,
>> + sync::{
>> + Arc,
>> + Mutex, //
>> + },
>> + time::Delta, //
>> +};
>> +
>> +use crate::{
>> + driver::Bar0,
>> + mm::VramAddress,
>> + regs, //
>> +};
>> +
>> +/// TLB manager for GPU translation buffer operations.
>> +#[pin_data]
>> +pub(crate) struct Tlb {
>> + bar: Arc<Devres<Bar0>>,
>> + /// TLB flush serialization lock: This lock is acquired during the
>> + /// DMA fence signalling critical path. It must NEVER be held across any
>> + /// reclaimable CPU memory allocations because the memory reclaim path can
>> + /// call `dma_fence_wait()`, which would deadlock with this lock held.
>> + #[pin]
>> + lock: Mutex<()>,
>> +}
>> +
>> +impl Tlb {
>> + /// Create a new TLB manager.
>> + pub(super) fn new(bar: Arc<Devres<Bar0>>) -> impl PinInit<Self> {
>> + pin_init!(Self {
>> + bar,
>> + lock <- new_mutex!((), "tlb_flush"),
>> + })
>> + }
>> +
>> + /// Flush the GPU TLB for a specific page directory base.
>> + ///
>> + /// This invalidates all TLB entries associated with the given PDB address.
>> + /// Must be called after modifying page table entries to ensure the GPU sees
>> + /// the updated mappings.
>> + pub(crate) fn flush(&self, pdb_addr: VramAddress) -> Result {
>
> This landed on my list randomly, so I took a look.
>
> Wouldn’t you want to virtualize the invalidation based on your device?
> For example, what if you need to register interface changes on future hardware?

Good point, for future hardware it indeed makes sense. I will do that.
> Or, if you’re a VF, can you even do MMIO?

For VFs, TLB flush is typically done directly via VF's BAR0 MMIO. If there is
VF-specific handling for any reason, that should probably be a different
interface than a per-chip interface (more specifically, checking if Nova is
booted on a VF).

thanks,

--
Joel Fernandes