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Re: [Qemu-ppc] [PATCH v2] linux-user: ppc64: don't use volatile register

From: Richard Henderson
Subject: Re: [Qemu-ppc] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall
Date: Thu, 26 Jul 2018 22:20:11 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 07/26/2018 08:13 PM, David Gibson wrote:
> On Thu, Jul 26, 2018 at 01:56:19PM +0530, Shivaprasad G Bhat wrote:
>> r11 is a volatile register on PPC as per calling conventions.
>> The safe_syscall code uses it to check if the signal_pending
>> is set during the safe_syscall. When a syscall is interrupted
>> on return from signal handling, the r11 might be corrupted
>> before we retry the syscall leading to a crash. The registers
>> r0-r13 are not to be used here as they have
>> volatile/designated/reserved usages. Change the code to use
>> r14 which is non-volatile and is appropriate for local use in
>> safe_syscall.
>> Steps to reproduce:
>> On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
>> Attempt Ctrl-C, the issue is reproduced.
>> Reference:
>> https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG
>> Signed-off-by: Shivaprasad G Bhat <address@hidden>
>> Tested-by: Laurent Vivier <address@hidden>
>> Reviewed-by: Laurent Vivier <address@hidden>
> Applied to ppc-for-3.0.

No please!  r14 is a call-saved register and needs saving before clobbering.


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