lib: sbi: Rework misaligned vector load/store

Fix the following issues with misaligned vector load/store:

a. Stack overflow: the mask[VLEN_MAX / 8] variable consumes 8K stack
space, given VLEN_MAX=65536, overflowing the default-sized stack.
There's no need to fetch the whole mask in one go, instead, make it
on-demand. Use a 128-byte mask as local buffer to hold the sliding
window of mask. For rvv load, this is allowed -- from the spec:

  "The destination vector register group for a masked vector
   instruction cannot overlap the source mask register (v0),
   unless the destination vector register is being written with
   a mask value (e.g., compares) or the scalar result of a reduction"

We don't need to worry about the mask getting overwritten.

b. Maintain the value of vstart upon abort (uptrap) to avoid duplicate
work. After fault resolution, the instruction can restart from the
faulting vstart. For Fault-Only-First loads, reset vstart to 0, as
previously done so, to conform to spec.

c. Explicitly set VS dirty in VSSTATUS with SET_VS_DIRTY() if faulting
from V=1, and if any vector register, including vstart/vl/vtype, gets
changed in the handler. It can add 1 unnecessary op to set VS dirty
in M/SSTATUS (not VSSTATUS), where the HW already did, but for code
simplicity, do it anyway. The overhead should be negligible.

Signed-off-by: Bo Gan <ganboing@gmail.com>
Tested-by: Anirudh Srinivasan <asrinivasan@oss.tenstorrent.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260609060024.706-5-ganboing@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

include/sbi/sbi_vector.h

@@ -20,6 +20,12 @@ struct sbi_vector_context {
 	uint8_t vregs[];
 };
 
+#define SET_VS_DIRTY(regs) do {				\
+	if (sbi_regs_from_virt(regs))			\
+		csr_set(CSR_VSSTATUS, MSTATUS_VS);	\
+	regs->mstatus |= MSTATUS_VS;			\
+} while(0)
+
 #ifdef OPENSBI_CC_SUPPORT_VECTOR
 void sbi_vector_save(struct sbi_vector_context *dst);
 void sbi_vector_restore(const struct sbi_vector_context *src);