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When we create sparse resources the first thing we do is a NULL bind
on them, as the Vulkan spec mandates certain behavior even for unbound
sparse resources. We do this with the minimal effort possible: if we
can get away with marking an L2 pointer as NULL in the L3 table, we
just do it and return, instead of going all the way to creating L1
tables and marking all the final entries as NULL.
The strategy we were using had a bug that could lead to previously
created NULL entries not being marked as NULL anymore. Let's give an
example:
(before proceeding, keep in mind that a NULL entry in the L3 and L2
tables has bit 1 set, it does *not* have the value 0)
- Create a 64mb buffer that uses an entire L1 table (needs to be
properly aligned), which triggers a NULL bind.
- Our algorithm will just set the L3 entry (pointing to the L2
table) as NULL.
- Create a 64kb buffer that uses the same L2 table (but a different
L1 table).
- The NULL bind triggered won't do anything as the L2 table is
already NULL.
- Bind the first buffer to actual memory. This will end up creating
the L2 table and the L1 table. The only entry we will set in the L2
table will be the one pointing to the L1 table. All the other
values will be 0 (so they won't have neither the NULL or Invalid
bits set: access to them will lead to page faults).
- Try to use the second buffer, which is still unbound. It was
relying on the fact that its L2 table pointer was NULL, but now
it's not anymore, so the page walker will fetch the L1 entries in
the L2 table and they will all be zero instead of having the NULL
bit set.
The fix is pretty simple: whenever we create a new L2 table, set every
entry to NULL (except the one we're about to set to non-NULL). This
preserves behavior for every other NULL resource relying on the L3
entry being set to NULL.
We don't need to do this for the L1 table because its entries are
different and instead of having bits to signal NULL entries we have
a special TR-TT register that we can set that gets compared to check
if an entry is NULL, and we conveniently program it to 0: see
ANV_TRTT_L1_NULL_TILE_VAL.
I am not aware of any real workloads that are triggering this
behavior, I found this issue while investigating something else,
running a custom sparse program in our pre-silicon environment, and it
told us about the page faults.
Cc: mesa-stable
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30953>
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`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library ====================================================== Source ------ This repository lives at https://gitlab.freedesktop.org/mesa/mesa. Other repositories are likely forks, and code found there is not supported. Build & install --------------- You can find more information in our documentation (`docs/install.rst <https://docs.mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.rst <https://docs.mesa3d.org/meson.html>`_): .. code-block:: sh $ meson setup build $ ninja -C build/ $ sudo ninja -C build/ install Support ------- Many Mesa devs hang on IRC; if you're not sure which channel is appropriate, you should ask your question on `OFTC's #dri-devel <irc://irc.oftc.net/dri-devel>`_, someone will redirect you if necessary. Remember that not everyone is in the same timezone as you, so it might take a while before someone qualified sees your question. To figure out who you're talking to, or which nick to ping for your question, check out `Who's Who on IRC <https://dri.freedesktop.org/wiki/WhosWho/>`_. The next best option is to ask your question in an email to the mailing lists: `mesa-dev\@lists.freedesktop.org <https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_ Bug reports ----------- If you think something isn't working properly, please file a bug report (`docs/bugs.rst <https://docs.mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.rst <https://docs.mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.