Hi John,
On 10/20/2025 4:59 PM, John Hubbard wrote:
> On 10/20/25 11:55 AM, Joel Fernandes wrote:
>> Add data structures and helpers for page table management. Uses
>> bitfield for cleanly representing and accessing the bitfields in the
>> structures.
>>
> ...
>> +bitfield! {
>> + pub(crate) struct Pte(u64), "Page Table Entry (PTE) to map virtual
>> pages to physical frames." {
>> + 0:0 valid as bool; // (1 = valid for PTEs)
>> + 1:1 privilege as bool; // P - Privileged/kernel-only
>> access
>> + 2:2 read_only as bool; // RO - Write protection
>> + 3:3 atomic_disable as bool; // AD - Disable atomic ops
>> + 4:4 encrypted as bool; // E - Encryption enabled
>> + 39:8 frame_number as u64; // PA[39:8] - Physical frame
>> number (32 bits)
>> + 41:40 aperture as u8 => AperturePte; // Memory aperture
>> type.
>> + 42:42 volatile as bool; // VOL - Volatile flag
>> + 50:43 kind as u8; // K[7:0] - Compression/tiling
>> kind
>> + 63:51 comptag_line as u16; // CTL[12:0] - Compression tag
>> line
>> + }
>> +}
>
> Hi Joel,
>
> What GPUs is this good for? I ask because I seem to recall that
> the format has changed over the years, on a per-GPU-architecture
> basis...
Yes, there's different format versions.
This patch supports all Turing and later GPUs because all GPUs including Hopper+
are backward compatible with version 1. However they wont be able to use the
version 2 and 3 features with only this patch.
I kind of intentionally did this for a first cut. But yes, I could split it into
versions. The 3 MMU structures (PTE, PDE and Dual PDE) are versioned. Version 2
is Turing and later. Hopper+ is when Version 3 got introduced and it is also
backward compatible with Version 2.
We could eventually support versions 2 and 3 (instead of just version 1 as I am
doing), however my working MMU translation prototype is based on version 1 (I
did not have to configure anything in the MMU to switch versions, this was
default).
There are a couple of options:
1. For starters, support only version 1. Drawback is, when/if we want to use
version 2 and 3 features, it may require some rework.
2. Have the following hierarchy:
mm/types.rs - all common structures (stuff that is generic like Pfn).
mm/types/ver1.rs - Version 1 specific types.
mm/types/ver2.rs - Version 2 specific types.
mm/types/ver3.rs - Version 3 specific types.
The advantage of this is it keeps the structs namespaced. So it'd be
nova_core::mm:types::ver2::Pte or nova_core::mm:types::ver3::Pte. And the
nova-core MMU code can pick the correct version.
3. Keep the single file types.rs and just suffix the structs with version
numbers. This is attractive because there are only 3 types that have version
flavors (pte, pde and dual pde). So instead of Pte, we would have PteVersion1,
PteVersion2 etc, and a helper abstraction can pick the correct struct.
4. Any of the options 1-3, but dropping version 1 since Turing+ supports version
2 and later. I do have to figure out how to configure the MMU to use a specific
version (which is reasonable).
5. Your options here.
Btw, I used Nouveau as a reference as well, so likely Nouveau doesn't support
version 2 and 3 features. Not that that matters (we should support newer
features in nova-core), but just thought I'd mention it.
Other thoughts?
thanks,
- Joel
