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Re: [Qemu-devel] [PATCH v3] linux-user: ppc64: don't use volatile regist
Re: [Qemu-devel] [PATCH v3] linux-user: ppc64: don't use volatile register during safe_syscall
Mon, 30 Jul 2018 21:16:35 +0200
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Le 30/07/2018 à 14:44, Richard Henderson a écrit :
> On 07/30/2018 06:09 AM, 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.
>> Use SP+16 which is a slot for LR, for save/restore of previous value
>> of r14. SP+16 can be used, as LR is preserved across the syscall.
>> Steps to reproduce:
>> On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
>> Attempt Ctrl-C, the issue is reproduced.
>> Signed-off-by: Shivaprasad G Bhat <address@hidden>
> This does fix the bug, so
> Tested-by: Richard Henderson <address@hidden>
> But we can do slightly better:
>> @@ -49,7 +49,8 @@ safe_syscall_base:
>> * and returns the result in r3
>> * Shuffle everything around appropriately.
>> - mr 11, 3 /* signal_pending */
>> + std 14, 16(1) /* Preserve r14 in SP+16 */
> Above this context, we have a .cfi_startproc directive, which indicates we are
> providing unwind information for this function (trivial up to this point).
> To preserve that, you need to add
> .cfi_offset 14, 16
> right here, indicating r14 is saved 16 bytes "up" the stack frame.
> Since this portion of the stack frame is allocated by the caller, this saved
> value remains valid through the end of the function, so we do not need to do
> anything at the points where the value is restored.
>> + ld 14, 16(1) /* restore r14 to its original value */
>> /* code path when we did execute the syscall */
> Swap these two lines.
> With those two changes,
> Reviewed-by: Richard Henderson <address@hidden>
Tested-by: Laurent Vivier <address@hidden>
Reviewed-by: Laurent Vivier <address@hidden>
I think this patch should go into the next -rc because it fixes a lot of
failures in the LTP on ppc64 host (hundreds of failure...).
David, if you want, I can take it through the linux-user tree.