Re: [PATCH v9 28/31] common-user: Add safe syscall handling for loongarch64 hosts

[WANG Xuerui <i.qemu@xen0n.name>]

Hi Richard,

On 2021/12/15 03:29, Richard Henderson wrote:
> On 12/14/21 12:01 AM, WANG Xuerui wrote:
>> +        move    $t0, $a0        /* signal_pending pointer */
> ...
>> +safe_syscall_start:
>> +        /* If signal_pending is non-zero, don't do the call */
>> +        ld.w    $t1, $t0, 0
>> +        bnez    $t1, 2f
>> +        syscall 0
>
> We need a non-syscall clobbered register for signal_pending, per the
> bug fixed in 5d9f3ea0817215ad4baac5aa30414e9ebbaaf0d6.
>
> In the case of riscv, because of the way exceptions are delivered,
> there are no syscall-clobbered registers (by the time syscall is
> distinguished from interrupt, all registers have been saved).
>
> In the case of mips, there are no non-syscall-clobbered registers that
> are not also call-saved or syscall arguments, so I had to allocate a
> stack frame and save/restore s0.
>
> For loongarch64, according to glibc,
>
> #define __SYSCALL_CLOBBERS \
>   "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7", "$t8", "memory"
>
> which does suggest that a6 is unused, saved across the syscall, and
> also call-clobbered (so we don't have to allocate a stack frame).
>
> I've had a browse through the loongarch kernel code and that seems to
> be all true. (Curiously, loongarch restores more registers than it
> saves on the way out of handle_syscall.  There may be a subtle reason
> for that, or room for improvement.)

Of course I completely forgot the fact that LoongArch looks more like
MIPS than RISC-V in kernel land (facepalm)

I've checked the LoongArch kernel sources too and yeah using a6 is ideal
and unlikely to break in the future (we're not allowing any more
7-argument syscalls into the kernel after all). I've just sent v10 with
some other minor changes.

>
>
> r~