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Improve PMU extension with snapshot feature
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Snapshot of PMU state allows the supervisor to read the counter values
or scountovf without any traps. This is mostly applicable for trap n emulate
scenarios in virtualization where the hypervisor traps for these CSRs.

In an non-virtualized world, the cost of CSR access vs dram access needs
to be considered.

Signed-off-by: Atish Patra <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
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@atishp04
atishp04 committed Jun 1, 2023
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108 changes: 98 additions & 10 deletions riscv-sbi.adoc
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Expand Up @@ -74,6 +74,7 @@ https://creativecommons.org/licenses/by/4.0/.
* Added SBI nested acceleration extension
* Added common description for a virtual HART
* Added SBI steal-time accounting extension
* Added SBI PMU snapshot extension

=== Version 1.0.0

Expand Down Expand Up @@ -1654,18 +1655,28 @@ The bit definitions of the `start_flags` parameter are shown in the
.PMU Counter Start Flags
[cols="3,1,2", width=90%, align="center", options="header"]
|===
| Flag Name | Bits | Description
| SBI_PMU_START_SET_INIT_VALUE | 0:0 | Set the value of counters
based on the `initial_value`
parameter
| *RESERVED* | 1:(XLEN-1) | All non-zero values are
reserved for future use
| Flag Name | Bits | Description
| SBI_PMU_START_SET_INIT_VALUE | 0:0 | Set the value of counters
based on the `initial_value`
parameter
| SBI_PMU_START_FLAG_INIT_SNAPSHOT | 1:1 | Initialize the given counters
from shared memory if
available.
| *RESERVED* | 2:(XLEN-1) | Reserved for future use
|===

NOTE: When SBI_PMU_START_SET_INIT_VALUE is not set in `start_flags`,
the counter value will not be modified and event counting will start
from current counter value.

The shared memory address must be set during boot via
`sbi_pmu_snapshot_set_shmem` before the `SBI_PMU_START_FLAG_INIT_SNAPSHOT`
flag may be used. The SBI implementation must initialize all the given valid
counters (to be started) from the value set in the shared snapshot memory.

NOTE: `SBI_PMU_START_SET_INIT_VALUE` and `SBI_PMU_START_FLAG_INIT_SNAPSHOT` are
mutually exclusive as the former is only valid for a single counter.

The possible error codes returned in `sbiret.error` are shown in the
<<table_pmu_counter_start_errors>> below.

Expand All @@ -1676,6 +1687,9 @@ The possible error codes returned in `sbiret.error` are shown in the
| Error code | Description
| SBI_SUCCESS | counter started successfully.
| SBI_ERR_INVALID_PARAM | set of counters has at least one invalid counter.
or the snapshot address is not configured and
`SBI_PMU_START_FLAG_INIT_SNAPSHOT` is set in the
flags.
| SBI_ERR_ALREADY_STARTED | set of counters includes at least one counter which
is already started.
|===
Expand All @@ -1698,12 +1712,24 @@ definitions of the `stop_flags` parameter are shown in the
.PMU Counter Stop Flags
[cols="3,1,2", width=90%, align="center", options="header"]
|===
| Flag Name | Bits | Description
| SBI_PMU_STOP_FLAG_RESET | 0:0 | Reset the counter to event mapping.
| *RESERVED* | 1:(XLEN-1) | All non-zero values are reserved
for future use
| Flag Name | Bits | Description
| SBI_PMU_STOP_FLAG_RESET | 0:0 | Reset the counter to event
mapping.
| SBI_PMU_STOP_FLAG_TAKE_SNAPSHOT | 1:1 | Save a snapshot of the given
counter's values in the shared
memory if available.
| *RESERVED* | 2:(XLEN-1) | Reserved for future use
|===


The shared memory address must be set during boot via
`sbi_pmu_snapshot_set_shmem` before the `SBI_PMU_STOP_FLAG_TAKE_SNAPSHOT` flag
may be used. The SBI implementation must save the current value of all the
stopped counters in the shared memory if `SBI_PMU_STOP_FLAG_TAKE_SNAPSHOT` is
set. The values corresponding to all other counters must not be modified. The
SBI implementation must additionally update the overflown counter bitmap in the
shared memory.

The possible error codes returned in `sbiret.error` are shown in the
<<table_pmu_counter_stop_errors>> below.

Expand All @@ -1714,6 +1740,9 @@ The possible error codes returned in `sbiret.error` are shown in the
| Error code | Description
| SBI_SUCCESS | counter stopped successfully.
| SBI_ERR_INVALID_PARAM | set of counters has at least one invalid counter.
Or the snapshot address is not configured and
`SBI_PMU_STOP_FLAG_TAKE_SNAPSHOT` is set in the
flags.
| SBI_ERR_ALREADY_STOPPED | set of counters includes at least one counter which
is already stopped.
|===
Expand Down Expand Up @@ -1766,6 +1795,64 @@ The possible error codes returned in `sbiret.error` are shown in
or an invalid counter.
|===

=== Function: Enable PMU snapshot feature (FID #7)

[source, C]
----
struct sbiret sbi_pmu_snapshot_set_shmem(unsigned long shmem_phys_lo,
unsigned long shmem_phys_hi)
----

Set shared memory area for PMU state snapshot. The `shmem_phys_lo` specifies
the lower XLEN bits and `shmem_phys_hi` specifies the upper XLEN bits of the
shared memory physical address. The `shmem_phys_lo` MUST be 4096 bytes
(i.e. page) aligned. The shared memory size must be 4096 bytes. The layout of
the shared memory is described in <<table_snapshot_shmem_layout>>.

[#table_snapshot_shmem_layout]
.SBI PMU Snapshot shared memory layout
[cols="5,2,2,5", width=90%, align="center", options="header"]
|===
| Name | Offset | Size | Description
| counter_overflow_bitmap | 0x0000 | 8 | A bitmap of all logical overflown
counters. This is valid only if
the `Sscofpmf` ISA extension is
available. Otherwise, it must be
zero.
| counter_values | 0x0008 | 512 | An array of 64-bit logical
counters where each index
represents the value of each
logical counter associated with
hardware/firmware.
| Reserved | 0x0208 | 3576 | Reserved for future use
|===

Any future revisions to this structure should be made in a backward compatible
manner and will be associated with an SBI version.

This function should be invoked only once per hart at boot time. Once
configured, the SBI implementation has read/write access to the shared memory
when `sbi_pmu_counter_stop` is invoked with the
`SBI_PMU_STOP_FLAG_TAKE_SNAPSHOT` flag set. The SBI implementation has read
only access when `sbi_pmu_counter_start` is invoked with the
`SBI_PMU_START_FLAG_INIT_SNAPSHOT` flag set. The SBI implementation must not
access this memory any other time.

The possible error codes returned in `sbiret.error` are shown in
<<table_pmu_snapshot_set_shmem_errors>> below.

[#table_pmu_snapshot_set_shmem_errors]
.PMU Setup Snapshot Area Errors
[cols="2,3", width=90%, align="center", options="header"]
|===
| Error code | Description
| SBI_SUCCESS | firmware counter read successfully.
| SBI_ERR_INVALID_ADDRESS | The shared memory pointed to by the `shmem_phys_lo`
and `shmem_phys_hi` parameters is not writable or
does not satisfy other requirements of
<<_shared_memory_physical_address_range_parameter>>.
|===

=== Function Listing

[#table_pmu_function_list]
Expand All @@ -1780,6 +1867,7 @@ The possible error codes returned in `sbiret.error` are shown in
| sbi_pmu_counter_stop | 0.3 | 4 | 0x504D55
| sbi_pmu_counter_fw_read | 0.3 | 5 | 0x504D55
| sbi_pmu_counter_fw_read_hi | 2.0 | 6 | 0x504D55
| sbi_pmu_snapshot_set_shmem | 2.0 | 7 | 0x504D55
|===

== Debug Console Extension (EID #0x4442434E "DBCN")
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