Re: [tip:x86/asm] x86/asm/entry/64: Remove unused thread_struct::usersp

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> Assuming this does not fix the regression, could you apply the 
> minimal patch below - which reverts the old_rsp handling change.

Assuming this solves the regression (it really should, it's now 
equivalent to a full revert minus comments):

> @@ -395,6 +398,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>  	/*
>  	 * Switch the PDA and FPU contexts.
>  	 */
> +	prev->usersp = this_cpu_read(old_rsp);
> +	this_cpu_write(old_rsp, next->usersp);
>  	this_cpu_write(current_task, next_p);
>  
>  	/*

can you confirm that your guest (sometimes) uses SYSENTER to do 
syscalls?

If yes then my theory is that we broke SYSENTER (or SYSEXIT) support - 
and that this would not be visible in our normal tests of KVM because 
SYSCALL is used most of the time.

Thanks,

	Ingo