netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 99 insertions, 0 deletions

diff --git a/Documentation/gpu/msm-preemption.rst b/Documentation/gpu/msm-preemption.rst
new file mode 100644
index 000000000..d768ca09f
--- /dev/null
+++ b/Documentation/gpu/msm-preemption.rst
@@ -0,0 +1,99 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+:orphan:
+
+==============
+MSM Preemption
+==============
+
+Preemption allows Adreno GPUs to switch to a higher priority ring when work is
+pushed to it, reducing latency for high priority submissions.
+
+When preemption is enabled 4 rings are initialized, corresponding to different
+priority levels. Having multiple rings is purely a software concept as the GPU
+only has registers to keep track of one graphics ring.
+The kernel is able to switch which ring is currently being processed by
+requesting preemption. When certain conditions are met, depending on the
+priority level, the GPU will save its current state in a series of buffers,
+then restores state from a similar set of buffers specified by the kernel. It
+then resumes execution and fires an IRQ to let the kernel know the context
+switch has completed.
+
+This mechanism can be used by the kernel to switch between rings. Whenever a
+submission occurs the kernel finds the highest priority ring which isn't empty
+and preempts to it if said ring is not the one being currently executed. This is
+also done whenever a submission completes to make sure execution resumes on a
+lower priority ring when a higher priority ring is done.
+
+Preemption levels
+-----------------
+
+Preemption can only occur at certain boundaries. The exact conditions can be
+configured by changing the preemption level, this allows to compromise between
+latency (ie. the time that passes between when the kernel requests preemption
+and when the SQE begins saving state) and overhead (the amount of state that
+needs to be saved).
+
+The GPU offers 3 levels:
+
+Level 0
+  Preemption only occurs at the submission level. This requires the least amount
+  of state to be saved as the execution of userspace submitted IBs is never
+  interrupted, however it offers very little benefit compared to not enabling
+  preemption of any kind.
+
+Level 1
+  Preemption occurs at either bin level, if using GMEM rendering, or draw level
+  in the sysmem rendering case.
+
+Level 2
+  Preemption occurs at draw level.
+
+Level 1 is the mode that is used by the msm driver.
+
+Additionally the GPU allows to specify a `skip_save_restore` option. This
+disables the saving and restoring of all registers except those relating to the
+operation of the SQE itself, reducing overhead. Saving and restoring is only
+skipped when using GMEM with Level 1 preemption. When enabling this userspace is
+expected to set the state that isn't preserved whenever preemption occurs which
+is done by specifying preamble and postambles. Those are IBs that are executed
+before and after preemption.
+
+Preemption buffers
+------------------
+
+A series of buffers are necessary to store the state of rings while they are not
+being executed. There are different kinds of preemption records and most of
+those require one buffer per ring. This is because preemption never occurs
+between submissions on the same ring, which always run in sequence when the ring
+is active. This means that only one context per ring is effectively active.
+
+SMMU_INFO
+  This buffer contains info about the current SMMU configuration such as the
+  ttbr0 register. The SQE firmware isn't actually able to save this record.
+  As a result SMMU info must be saved manually from the CP to a buffer and the
+  SMMU record updated with info from said buffer before triggering
+  preemption.
+
+NON_SECURE
+  This is the main preemption record where most state is saved. It is mostly
+  opaque to the kernel except for the first few words that must be initialized
+  by the kernel.
+
+SECURE
+  This saves state related to the GPU's secure mode.
+
+NON_PRIV
+  The intended purpose of this record is unknown. The SQE firmware actually
+  ignores it and therefore msm doesn't handle it.
+
+COUNTER
+  This record is used to save and restore performance counters.
+
+Handling the permissions of those buffers is critical for security. All but the
+NON_PRIV records need to be inaccessible from userspace, so they must be mapped
+in the kernel address space with the MSM_BO_MAP_PRIV flag.
+For example, making the NON_SECURE record accessible from userspace would allow
+any process to manipulate a saved ring's RPTR which can be used to skip the
+execution of some packets in a ring and execute user commands with higher
+privileges.