[Xen-devel] RE: [PATCH] Fixing stack alignment in x86-64 Xen

["Nakajima, Jun" <jun.nakajima@xxxxxxxxx>]

Andi Kleen wrote:
> "Nakajima, Jun" <jun.nakajima@xxxxxxxxx> writes:
> 
>> Long mode needs to align the stack on a 16-byte boundary. Recent
>> changes to Xen broke the requirement, and x86-64 XenLinux stopped
>> booting. The attached fixes the problem.
> 
> Normally it should be only needed in user space for saving FP
> registers. The kernel and Xen which should not do this probably don't
> need it. 

Normally they don't care. The problem was that Xen set rsp0 in TSS on a
8-byte boundary and upon interrupts in Ring3 (XenLinux or user
processes) the processor started pushing registers (ss, rsp, rflags, ..,
rip) on the 16-byte boundary in Xen.  

The recent optimization reset_stack_and_jump() code needs to know the
exact address of the interrupt stack (because it resets %rsp), and
calculates it based on the value that Xen set (i.e. 8-byte boundary).
Since the processor forces the rsp0 on a 16-byte boundary (i.e. moves it
down by 8 bytes), Xen sees a wrong stack when returning from the
interrupt.

Jun

> 
> Unless you save FP registers on the stack somewhere. Then I would
> rather fix that place only.
> 
> At least the main linux kernel does not try to keep the stack
> always 16byte aligned. There is even a gcc option to turn it off
> and it saves some code (and probably stack) size.
> 
> -Andi


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