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FreeRTOS-Kernel/portable/GCC/RISC-V/portContext.h
Gaurav-Aggarwal-AWS c8d31ddcff
Use saved mstatus for FPU/VPU state determination (#1330) 
According to the RISC-V Privileged Architecture Specification (20211203),
writing Initial or Clean to the FS field of mstatus may result in the FS
value getting set to Dirty in some implementations. This means we cannot
rely on reading back the same FS value after writing to mstatus.

Previously, the context restore code would:
1. Write an FS value to mstatus
2. Read mstatus again at a later point
3. Use the read FS value to determine FPU status

This change updates the context restore code to use the mstatus value
from the saved context instead of re-reading mstatus after writing to
it. This required chaning the location of the mstatus slot in the
context.

Fixes: https://github.com/FreeRTOS/FreeRTOS-Kernel/issues/1327

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
2025-11-03 12:32:08 +05:30

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/*
* FreeRTOS Kernel <DEVELOPMENT BRANCH>
* Copyright (C) 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* SPDX-License-Identifier: MIT
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*
* https://www.FreeRTOS.org
* https://github.com/FreeRTOS
*
*/
#ifndef PORTCONTEXT_H
#define PORTCONTEXT_H
#ifndef configENABLE_FPU
#define configENABLE_FPU 0
#endif
#ifndef configENABLE_VPU
#define configENABLE_VPU 0
#endif
#if __riscv_xlen == 64
#define portWORD_SIZE 8
#define store_x sd
#define load_x ld
#elif __riscv_xlen == 32
#define store_x sw
#define load_x lw
#define portWORD_SIZE 4
#else
#error Assembler did not define __riscv_xlen
#endif
#include "freertos_risc_v_chip_specific_extensions.h"
/* Only the standard core registers are stored by default. Any additional
* registers must be saved by the portasmSAVE_ADDITIONAL_REGISTERS and
* portasmRESTORE_ADDITIONAL_REGISTERS macros - which can be defined in a chip
* specific version of freertos_risc_v_chip_specific_extensions.h. See the
* notes at the top of portASM.S file. */
#ifdef __riscv_32e
#define portCONTEXT_SIZE ( 15 * portWORD_SIZE )
#define portCRITICAL_NESTING_OFFSET 14
#else
#define portCONTEXT_SIZE ( 31 * portWORD_SIZE )
#define portCRITICAL_NESTING_OFFSET 30
#endif
#if ( configENABLE_FPU == 1 )
/* Bit [14:13] in the mstatus encode the status of FPU state which is one of
* the following values:
* 1. Value: 0, Meaning: Off.
* 2. Value: 1, Meaning: Initial.
* 3. Value: 2, Meaning: Clean.
* 4. Value: 3, Meaning: Dirty.
*/
#define MSTATUS_FS_MASK 0x6000
#define MSTATUS_FS_INITIAL 0x2000
#define MSTATUS_FS_CLEAN 0x4000
#define MSTATUS_FS_DIRTY 0x6000
#define MSTATUS_FS_OFFSET 13
#ifdef __riscv_fdiv
#if __riscv_flen == 32
#define load_f flw
#define store_f fsw
#elif __riscv_flen == 64
#define load_f fld
#define store_f fsd
#else
#error Assembler did not define __riscv_flen
#endif
#define portFPU_REG_SIZE ( __riscv_flen / 8 )
#define portFPU_REG_COUNT 33 /* 32 Floating point registers plus one CSR. */
#define portFPU_REG_OFFSET( regIndex ) ( ( 2 * portWORD_SIZE ) + ( regIndex * portFPU_REG_SIZE ) )
#define portFPU_CONTEXT_SIZE ( portFPU_REG_SIZE * portFPU_REG_COUNT )
#else
#error configENABLE_FPU must not be set to 1 if the hardware does not have FPU
#endif
#endif
#if ( configENABLE_VPU == 1 )
/* Bit [10:9] in the mstatus encode the status of VPU state which is one of
* the following values:
* 1. Value: 0, Meaning: Off.
* 2. Value: 1, Meaning: Initial.
* 3. Value: 2, Meaning: Clean.
* 4. Value: 3, Meaning: Dirty.
*/
#define MSTATUS_VS_MASK 0x600
#define MSTATUS_VS_INITIAL 0x200
#define MSTATUS_VS_CLEAN 0x400
#define MSTATUS_VS_DIRTY 0x600
#define MSTATUS_VS_OFFSET 9
#ifndef __riscv_vector
#error configENABLE_VPU must not be set to 1 if the hardware does not have VPU
#endif
#endif
/*-----------------------------------------------------------*/
.extern pxCurrentTCB
.extern xISRStackTop
.extern xCriticalNesting
.extern pxCriticalNesting
/*-----------------------------------------------------------*/
.macro portcontexSAVE_FPU_CONTEXT
addi sp, sp, -( portFPU_CONTEXT_SIZE )
/* Store the FPU registers. */
store_f f0, portFPU_REG_OFFSET( 0 )( sp )
store_f f1, portFPU_REG_OFFSET( 1 )( sp )
store_f f2, portFPU_REG_OFFSET( 2 )( sp )
store_f f3, portFPU_REG_OFFSET( 3 )( sp )
store_f f4, portFPU_REG_OFFSET( 4 )( sp )
store_f f5, portFPU_REG_OFFSET( 5 )( sp )
store_f f6, portFPU_REG_OFFSET( 6 )( sp )
store_f f7, portFPU_REG_OFFSET( 7 )( sp )
store_f f8, portFPU_REG_OFFSET( 8 )( sp )
store_f f9, portFPU_REG_OFFSET( 9 )( sp )
store_f f10, portFPU_REG_OFFSET( 10 )( sp )
store_f f11, portFPU_REG_OFFSET( 11 )( sp )
store_f f12, portFPU_REG_OFFSET( 12 )( sp )
store_f f13, portFPU_REG_OFFSET( 13 )( sp )
store_f f14, portFPU_REG_OFFSET( 14 )( sp )
store_f f15, portFPU_REG_OFFSET( 15 )( sp )
store_f f16, portFPU_REG_OFFSET( 16 )( sp )
store_f f17, portFPU_REG_OFFSET( 17 )( sp )
store_f f18, portFPU_REG_OFFSET( 18 )( sp )
store_f f19, portFPU_REG_OFFSET( 19 )( sp )
store_f f20, portFPU_REG_OFFSET( 20 )( sp )
store_f f21, portFPU_REG_OFFSET( 21 )( sp )
store_f f22, portFPU_REG_OFFSET( 22 )( sp )
store_f f23, portFPU_REG_OFFSET( 23 )( sp )
store_f f24, portFPU_REG_OFFSET( 24 )( sp )
store_f f25, portFPU_REG_OFFSET( 25 )( sp )
store_f f26, portFPU_REG_OFFSET( 26 )( sp )
store_f f27, portFPU_REG_OFFSET( 27 )( sp )
store_f f28, portFPU_REG_OFFSET( 28 )( sp )
store_f f29, portFPU_REG_OFFSET( 29 )( sp )
store_f f30, portFPU_REG_OFFSET( 30 )( sp )
store_f f31, portFPU_REG_OFFSET( 31 )( sp )
csrr t0, fcsr
store_x t0, portFPU_REG_OFFSET( 32 )( sp )
.endm
/*-----------------------------------------------------------*/
.macro portcontextRESTORE_FPU_CONTEXT
/* Restore the FPU registers. */
load_f f0, portFPU_REG_OFFSET( 0 )( sp )
load_f f1, portFPU_REG_OFFSET( 1 )( sp )
load_f f2, portFPU_REG_OFFSET( 2 )( sp )
load_f f3, portFPU_REG_OFFSET( 3 )( sp )
load_f f4, portFPU_REG_OFFSET( 4 )( sp )
load_f f5, portFPU_REG_OFFSET( 5 )( sp )
load_f f6, portFPU_REG_OFFSET( 6 )( sp )
load_f f7, portFPU_REG_OFFSET( 7 )( sp )
load_f f8, portFPU_REG_OFFSET( 8 )( sp )
load_f f9, portFPU_REG_OFFSET( 9 )( sp )
load_f f10, portFPU_REG_OFFSET( 10 )( sp )
load_f f11, portFPU_REG_OFFSET( 11 )( sp )
load_f f12, portFPU_REG_OFFSET( 12 )( sp )
load_f f13, portFPU_REG_OFFSET( 13 )( sp )
load_f f14, portFPU_REG_OFFSET( 14 )( sp )
load_f f15, portFPU_REG_OFFSET( 15 )( sp )
load_f f16, portFPU_REG_OFFSET( 16 )( sp )
load_f f17, portFPU_REG_OFFSET( 17 )( sp )
load_f f18, portFPU_REG_OFFSET( 18 )( sp )
load_f f19, portFPU_REG_OFFSET( 19 )( sp )
load_f f20, portFPU_REG_OFFSET( 20 )( sp )
load_f f21, portFPU_REG_OFFSET( 21 )( sp )
load_f f22, portFPU_REG_OFFSET( 22 )( sp )
load_f f23, portFPU_REG_OFFSET( 23 )( sp )
load_f f24, portFPU_REG_OFFSET( 24 )( sp )
load_f f25, portFPU_REG_OFFSET( 25 )( sp )
load_f f26, portFPU_REG_OFFSET( 26 )( sp )
load_f f27, portFPU_REG_OFFSET( 27 )( sp )
load_f f28, portFPU_REG_OFFSET( 28 )( sp )
load_f f29, portFPU_REG_OFFSET( 29 )( sp )
load_f f30, portFPU_REG_OFFSET( 30 )( sp )
load_f f31, portFPU_REG_OFFSET( 31 )( sp )
load_x t0, portFPU_REG_OFFSET( 32 )( sp )
csrw fcsr, t0
addi sp, sp, ( portFPU_CONTEXT_SIZE )
.endm
/*-----------------------------------------------------------*/
.macro portcontexSAVE_VPU_CONTEXT
/* Un-reserve the space reserved for mstatus and epc. */
add sp, sp, ( 2 * portWORD_SIZE )
csrr t0, vlenb /* t0 = vlenb. vlenb is the length of each vector register in bytes. */
slli t0, t0, 3 /* t0 = vlenb * 8. t0 now contains the space required to store 8 vector registers. */
neg t0, t0
/* Store the vector registers in group of 8. */
add sp, sp, t0
vs8r.v v0, (sp) /* Store v0-v7. */
add sp, sp, t0
vs8r.v v8, (sp) /* Store v8-v15. */
add sp, sp, t0
vs8r.v v16, (sp) /* Store v16-v23. */
add sp, sp, t0
vs8r.v v24, (sp) /* Store v24-v31. */
/* Store the VPU CSRs. */
addi sp, sp, -( 4 * portWORD_SIZE )
csrr t0, vstart
store_x t0, 0 * portWORD_SIZE( sp )
csrr t0, vcsr
store_x t0, 1 * portWORD_SIZE( sp )
csrr t0, vl
store_x t0, 2 * portWORD_SIZE( sp )
csrr t0, vtype
store_x t0, 3 * portWORD_SIZE( sp )
/* Re-reserve the space for mstatus and epc. */
add sp, sp, -( 2 * portWORD_SIZE )
.endm
/*-----------------------------------------------------------*/
.macro portcontextRESTORE_VPU_CONTEXT
/* Un-reserve the space reserved for mstatus and epc. */
add sp, sp, ( 2 * portWORD_SIZE )
/* Restore the VPU CSRs. */
load_x t0, 0 * portWORD_SIZE( sp )
csrw vstart, t0
load_x t0, 1 * portWORD_SIZE( sp )
csrw vcsr, t0
load_x t0, 2 * portWORD_SIZE( sp )
load_x t1, 3 * portWORD_SIZE( sp )
vsetvl x0, t0, t1 /* vlen and vtype can only be updated by using vset*vl* instructions. */
addi sp, sp, ( 4 * portWORD_SIZE )
csrr t0, vlenb /* t0 = vlenb. vlenb is the length of each vector register in bytes. */
slli t0, t0, 3 /* t0 = vlenb * 8. t0 now contains the space required to store 8 vector registers. */
/* Restore the vector registers. */
vl8r.v v24, (sp)
add sp, sp, t0
vl8r.v v16, (sp)
add sp, sp, t0
vl8r.v v8, (sp)
add sp, sp, t0
vl8r.v v0, (sp)
add sp, sp, t0
/* Re-reserve the space for mstatus and epc. */
add sp, sp, -( 2 * portWORD_SIZE )
.endm
/*-----------------------------------------------------------*/
.macro portcontextSAVE_CONTEXT_INTERNAL
addi sp, sp, -portCONTEXT_SIZE
store_x x1, 2 * portWORD_SIZE( sp )
store_x x5, 3 * portWORD_SIZE( sp )
store_x x6, 4 * portWORD_SIZE( sp )
store_x x7, 5 * portWORD_SIZE( sp )
store_x x8, 6 * portWORD_SIZE( sp )
store_x x9, 7 * portWORD_SIZE( sp )
store_x x10, 8 * portWORD_SIZE( sp )
store_x x11, 9 * portWORD_SIZE( sp )
store_x x12, 10 * portWORD_SIZE( sp )
store_x x13, 11 * portWORD_SIZE( sp )
store_x x14, 12 * portWORD_SIZE( sp )
store_x x15, 13 * portWORD_SIZE( sp )
#ifndef __riscv_32e
store_x x16, 14 * portWORD_SIZE( sp )
store_x x17, 15 * portWORD_SIZE( sp )
store_x x18, 16 * portWORD_SIZE( sp )
store_x x19, 17 * portWORD_SIZE( sp )
store_x x20, 18 * portWORD_SIZE( sp )
store_x x21, 19 * portWORD_SIZE( sp )
store_x x22, 20 * portWORD_SIZE( sp )
store_x x23, 21 * portWORD_SIZE( sp )
store_x x24, 22 * portWORD_SIZE( sp )
store_x x25, 23 * portWORD_SIZE( sp )
store_x x26, 24 * portWORD_SIZE( sp )
store_x x27, 25 * portWORD_SIZE( sp )
store_x x28, 26 * portWORD_SIZE( sp )
store_x x29, 27 * portWORD_SIZE( sp )
store_x x30, 28 * portWORD_SIZE( sp )
store_x x31, 29 * portWORD_SIZE( sp )
#endif /* ifndef __riscv_32e */
load_x t0, xCriticalNesting /* Load the value of xCriticalNesting into t0. */
store_x t0, portCRITICAL_NESTING_OFFSET * portWORD_SIZE( sp ) /* Store the critical nesting value to the stack. */
#if( configENABLE_FPU == 1 )
csrr t0, mstatus
srl t1, t0, MSTATUS_FS_OFFSET
andi t1, t1, 3
addi t2, x0, 3
bne t1, t2, 1f /* If FPU status is not dirty, do not save FPU registers. */
portcontexSAVE_FPU_CONTEXT
1:
#endif
#if( configENABLE_VPU == 1 )
csrr t0, mstatus
srl t1, t0, MSTATUS_VS_OFFSET
andi t1, t1, 3
addi t2, x0, 3
bne t1, t2, 2f /* If VPU status is not dirty, do not save VPU registers. */
portcontexSAVE_VPU_CONTEXT
2:
#endif
csrr t0, mstatus
store_x t0, 1 * portWORD_SIZE( sp )
portasmSAVE_ADDITIONAL_REGISTERS /* Defined in freertos_risc_v_chip_specific_extensions.h to save any registers unique to the RISC-V implementation. */
#if( configENABLE_FPU == 1 )
/* Mark the FPU as clean, if it was dirty and we saved FPU registers. */
srl t1, t0, MSTATUS_FS_OFFSET
andi t1, t1, 3
addi t2, x0, 3
bne t1, t2, 3f
li t1, ~MSTATUS_FS_MASK
and t0, t0, t1
li t1, MSTATUS_FS_CLEAN
or t0, t0, t1
csrw mstatus, t0
3:
#endif
#if( configENABLE_VPU == 1 )
/* Mark the VPU as clean, if it was dirty and we saved VPU registers. */
srl t1, t0, MSTATUS_VS_OFFSET
andi t1, t1, 3
addi t2, x0, 3
bne t1, t2, 4f
li t1, ~MSTATUS_VS_MASK
and t0, t0, t1
li t1, MSTATUS_VS_CLEAN
or t0, t0, t1
csrw mstatus, t0
4:
#endif
load_x t0, pxCurrentTCB /* Load pxCurrentTCB. */
store_x sp, 0 ( t0 ) /* Write sp to first TCB member. */
.endm
/*-----------------------------------------------------------*/
.macro portcontextSAVE_EXCEPTION_CONTEXT
portcontextSAVE_CONTEXT_INTERNAL
csrr a0, mcause
csrr a1, mepc
addi a1, a1, 4 /* Synchronous so update exception return address to the instruction after the instruction that generated the exception. */
store_x a1, 0 ( sp ) /* Save updated exception return address. */
load_x sp, xISRStackTop /* Switch to ISR stack. */
.endm
/*-----------------------------------------------------------*/
.macro portcontextSAVE_INTERRUPT_CONTEXT
portcontextSAVE_CONTEXT_INTERNAL
csrr a0, mcause
csrr a1, mepc
store_x a1, 0 ( sp ) /* Asynchronous interrupt so save unmodified exception return address. */
load_x sp, xISRStackTop /* Switch to ISR stack. */
.endm
/*-----------------------------------------------------------*/
.macro portcontextRESTORE_CONTEXT
load_x t1, pxCurrentTCB /* Load pxCurrentTCB. */
load_x sp, 0 ( t1 ) /* Read sp from first TCB member. */
/* Load mepc with the address of the instruction in the task to run next. */
load_x t0, 0 ( sp )
csrw mepc, t0
/* Defined in freertos_risc_v_chip_specific_extensions.h to restore any registers unique to the RISC-V implementation. */
portasmRESTORE_ADDITIONAL_REGISTERS
/* Restore mstatus register. It is important to use t3 (and not t0) here as t3
* is not clobbered by portcontextRESTORE_VPU_CONTEXT and
* portcontextRESTORE_FPU_CONTEXT. */
load_x t3, 1 * portWORD_SIZE( sp )
csrw mstatus, t3
#if( configENABLE_VPU == 1 )
srl t1, t3, MSTATUS_VS_OFFSET
andi t1, t1, 3
addi t2, x0, 3
bne t1, t2, 5f /* If VPU status is not dirty, do not restore VPU registers. */
portcontextRESTORE_VPU_CONTEXT
5:
#endif /* ifdef portasmSTORE_VPU_CONTEXT */
#if( configENABLE_FPU == 1 )
srl t1, t3, MSTATUS_FS_OFFSET
andi t1, t1, 3
addi t2, x0, 3
bne t1, t2, 6f /* If FPU status is not dirty, do not restore FPU registers. */
portcontextRESTORE_FPU_CONTEXT
6:
#endif /* ifdef portasmSTORE_FPU_CONTEXT */
load_x t0, portCRITICAL_NESTING_OFFSET * portWORD_SIZE( sp ) /* Obtain xCriticalNesting value for this task from task's stack. */
load_x t1, pxCriticalNesting /* Load the address of xCriticalNesting into t1. */
store_x t0, 0 ( t1 ) /* Restore the critical nesting value for this task. */
load_x x1, 2 * portWORD_SIZE( sp )
load_x x5, 3 * portWORD_SIZE( sp )
load_x x6, 4 * portWORD_SIZE( sp )
load_x x7, 5 * portWORD_SIZE( sp )
load_x x8, 6 * portWORD_SIZE( sp )
load_x x9, 7 * portWORD_SIZE( sp )
load_x x10, 8 * portWORD_SIZE( sp )
load_x x11, 9 * portWORD_SIZE( sp )
load_x x12, 10 * portWORD_SIZE( sp )
load_x x13, 11 * portWORD_SIZE( sp )
load_x x14, 12 * portWORD_SIZE( sp )
load_x x15, 13 * portWORD_SIZE( sp )
#ifndef __riscv_32e
load_x x16, 14 * portWORD_SIZE( sp )
load_x x17, 15 * portWORD_SIZE( sp )
load_x x18, 16 * portWORD_SIZE( sp )
load_x x19, 17 * portWORD_SIZE( sp )
load_x x20, 18 * portWORD_SIZE( sp )
load_x x21, 19 * portWORD_SIZE( sp )
load_x x22, 20 * portWORD_SIZE( sp )
load_x x23, 21 * portWORD_SIZE( sp )
load_x x24, 22 * portWORD_SIZE( sp )
load_x x25, 23 * portWORD_SIZE( sp )
load_x x26, 24 * portWORD_SIZE( sp )
load_x x27, 25 * portWORD_SIZE( sp )
load_x x28, 26 * portWORD_SIZE( sp )
load_x x29, 27 * portWORD_SIZE( sp )
load_x x30, 28 * portWORD_SIZE( sp )
load_x x31, 29 * portWORD_SIZE( sp )
#endif /* ifndef __riscv_32e */
addi sp, sp, portCONTEXT_SIZE
mret
.endm
/*-----------------------------------------------------------*/
#endif /* PORTCONTEXT_H */
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