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Re: [PATCH 3/3] aarch64: Save and restore SVE registers in

On 08/02/2018 04:18 PM, Carlos O'Donell wrote:
On 08/02/2018 09:32 AM, Szabolcs Nagy wrote:
On 02/08/18 14:05, Richard Henderson wrote:
On 08/02/2018 03:29 AM, Florian Weimer wrote:
On 08/02/2018 12:23 AM, wrote:
+ * For functions conforming to the procedure call standard as
+ * amended for SVE support (ARM_100986_0000_00_en (SVEpcs 00bet1)),
+ * we must save the entire contents of Z0-Z7 as well as P0-P3.
+ */

What's the worst-case additional stack usage due to this change?

The current architectural maximum vector size is 256 bytes,
so 2176 bytes total.

that limit can increase in the future and saving 8 z regs is not enough,
you need to save all 32 if the resolver code may touch the 32 fp regs.
(so with 256byte regs it's 8k + 128bytes for 4 pregs)

AArch64 has PTHREAD_MIN_STACK of 128KiB so this should be more than enough
to save and restore SVE registers for some simple function calling.

On the other hand, there is also this:

/* Minimum stack size for a signal handler.  */
#define MINSIGSTKSZ     5120

/* System default stack size.  */
#define SIGSTKSZ        16384

Since the kernel will also push >8KiB on the signal handler stack, SIGSTKSZ is no longer sufficient.


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