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Use saved mstatus for FPU/VPU state determination (#1330)

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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>
This commit is contained in:
Gaurav-Aggarwal-AWS 2025-11-03 12:32:08 +05:30 • committed by GitHub
parent 8b63f94d8d
commit c8d31ddcff
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3 changed files with 35 additions and 34 deletions
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22
portable/GCC/RISC-V/portContext.h
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@ -322,17 +322,17 @@ store_x t0, portCRITICAL_NESTING_OFFSET * portWORD_SIZE( sp ) /* Store the criti
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 FPU registers. */
bne t1, t2, 2f /* If VPU status is not dirty, do not save VPU registers. */
portcontexSAVE_VPU_CONTEXT
2:
#endif
portasmSAVE_ADDITIONAL_REGISTERS /* Defined in freertos_risc_v_chip_specific_extensions.h to save any registers unique to the RISC-V implementation. */
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
@ -396,16 +396,17 @@ load_x sp, 0 ( t1 ) /* Read sp from first TCB member. */
load_x t0, 0 ( sp )
csrw mepc, t0
/* Restore mstatus register. */
load_x t0, 1 * portWORD_SIZE( sp )
csrw mstatus, 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 )
csrr t0, mstatus
srl t1, t0, MSTATUS_VS_OFFSET
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. */
@ -415,8 +416,7 @@ portasmRESTORE_ADDITIONAL_REGISTERS
#endif /* ifdef portasmSTORE_VPU_CONTEXT */
#if( configENABLE_FPU == 1 )
csrr t0, mstatus
srl t1, t0, MSTATUS_FS_OFFSET
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. */