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SubjectRe: [PATCH V3] MIPS: Add get_thread_area syscall
Hi Guoyun,

On Mon, Dec 16, 2019 at 10:17:51AM +0800, Guoyun Sun wrote:
> CRIU(https://criu.org/) tools will dump TLS(Thread Local Storage) by
> get_thread_area during checkpoint and restore TLS by set_thread_area during
> restore. without this syscall, criu restore will fail on MIPS platform
> because a variable with GCC __thread will be invalid.
>
> The following function will be called when criu restore
>
> static inline void restore_tls(tls_t *ptls) {
> asm volatile(
> "move $4, %0 \n"
> "li $2, "__stringify(__NR_set_thread_area)" \n"
> "syscall \n"
> :
> : "r"(*ptls)
> : "$4","$2","memory");
>
> the *ptls can be obtained by get_thread_area on MIPS platform when criu
> checkpoint. just like this:
>
> static inline void arch_get_tls(tls_t *ptls)
> {
> asm volatile(
> "move $4, %0 \n"
> "li $2, "__stringify(__NR_get_thread_area)" \n"
> "syscall \n"
> :
> : "r"(ptls)
> : "$4","$2","memory");
>
> }

Why would you need a syscall for this?

Why not retrieve the value using the rdhwr instruction just like every
other bit of code using thread-local storage does?

ie. something like:

static inline void arch_get_tls(tls_t *ptls)
{
asm("rdhwr %0, $29" : "=r"(*ptls));
}

Thanks,
Paul

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