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peripherals: 0.504

`vill` not set after reserved `vsetvli` instruction usage
Description of problem:
The ["AVL encoding" section of the RISC-V V Spec 1.0](https://github.com/riscv/riscv-isa-manual/blob/main/src/v-st-ext.adoc#avl-encoding) states that a `vsetvli  x0,x0,...` that changes VLMAX is reserved and "Implementations may set `vill`" in this case. QEMU does not set `vill` in this case. Doing so would help detect code generation issues and non-portable code.

Here is the quote from the spec:

> When `rs1=x0` and `rd=x0`, the instruction operates as if the current
> vector length in `vl` is used as the AVL, and the resulting value is
> written to `vl`, but not to a destination register.  This form can
> only be used when VLMAX and hence `vl` is not actually changed by the
> new SEW/LMUL ratio.  Use of the instruction with a new SEW/LMUL ratio
> that would result in a change of VLMAX is reserved.
> Use of the instruction is also reserved if `vill` was 1 beforehand.
> Implementations may set `vill` in either case.

Note, I have not checked QEMU's behaviour for the other case mentioned in the quote: "Use of the instruction is also reserved if `vill` was 1 beforehand".
Steps to reproduce:
1. Create `test.c`
```C
#include <assert.h>

/* Position of vill in vtype.  */

#define VILL_BIT (__riscv_xlen - 1)

/* Return true if vill is 1.  */

int vill_set_p ()
{
  __UINT64_TYPE__ vtype;
  asm volatile ("csrr %0, vtype" : "=r"(vtype));

  return (vtype >> VILL_BIT) & 1;
}

/* Return true if vill is 0.  */

int vill_clear_p ()
{
  return !vill_set_p ();
}

int main ()
{
  int vl;

  assert (vill_clear_p ());

  /* Valid: vl = VLMAX.  */
  asm volatile ("vsetvli %0,zero,e64,m8,ta,ma\n" : "=r"(vl));
  assert (vill_clear_p ());

  /* Valid: vl and VLMAX not changed.  */
  asm volatile ("vsetvli zero,zero,e64,m8,ta,ma\n");
  assert (vill_clear_p ());

  /* Reserved: Reduce VLMAX.  */
  asm volatile ("vsetvli zero,zero,e64,m1,ta,ma\n");
  assert (vill_set_p ());

  return 0;
}
```
2. Build `test.c` with `riscv32-unknown-elf-gcc test.c -o test -march=rv64gcv -mabi=lp64d`
3. Run qemu with `qemu-riscv64 -cpu rv64,v=true test`
4. The final assertion fails because executing the reserved `vsetvli` did not set `vill`
```
assertion "vill_set_p ()" failed: file "test.c", line 40, function: main
```