Re: [PATCH 1/3] x86: get rid of KERNEL_STACK_OFFSET

[Denys Vlasenko]

On 03/19/2015 04:43 PM, Andy Lutomirski wrote:
> On Thu, Mar 19, 2015 at 8:28 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>> On 03/18/2015 09:54 PM, Andy Lutomirski wrote:
>>> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>>>> PER_CPU_VAR(kernel_stack) was set up in a way where it points
>>>> five stack slots below the top of stack.
>>>>
>>>> Presumably, it was done to avoid one "sub $5*8,%rsp"
>>>> in syscall/sysenter code paths, where iret frame needs to be
>>>> created by hand.
>>>>
>>>> Ironically, none of them benefits from this optimization,
>>>> since all of them need to allocate additional data on stack
>>>> (struct pt_regs), so they still have to perform subtraction.
>>>>
>>>> This patch eliminates KERNEL_STACK_OFFSET.
>>>>
>>>> PER_CPU_VAR(kernel_stack) now points directly to top of stack.
>>>> pt_regs allocations are adjusted to allocate iret frame as well.
>>>> Hopefully we can merge it later with 32-bit specific
>>>> PER_CPU_VAR(cpu_current_top_of_stack) variable...
>>>>
>>>> Semi-mysterious expressions THREAD_INFO(%rsp,RIP) - "why RIP??"
>>>> are now replaced by more logical THREAD_INFO(%rsp,SIZEOF_PTREGS) -
>>>> "calculate thread_info's address using information that
>>>> rsp is SIZEOF_PTREGS bytes below the stack top".
>>>>
>>>> Net result in generated code is that constants in several insns
>>>> are changed.
>>>>
>>>> This change is necessary for changing struct pt_regs creation
>>>> in SYSCALL64 code path from MOV to PUSH instructions.
>>>>
>>>
>>> Would it be reasonable to break this up into two pieces: first, remove
>>> KERNEL_STACK_OFFSET from THREAD_INFO and related macros (i.e. make
>>> them relative to current_top_of_stack instead)
>>
>> PER_CPU(cpu_current_top_of_stack) exists only in 32 bits.
>> Can't use it in 64-bit code.
>>
>> PER_CPU(cpu_current_top_of_stack) becomes equal
>> to PER_CPU(kernel_stack) only after this patch.
> 
> You could have a #define that gives the top of the stack from asm, though.
> 
>>
>> I plan to clean up PER_CPU(cpu_current_top_of_stack)/
>> PER_CPU(kernel_stack)/PER_CPU(tss->sp0) mess
>> on top of my patches.
>>
>> First: this seems to be the easier way.
> 
> It's probably easier, but it's harder to review since you're making
> two change at once (changing the rsp loaded during syscall and
> changing the way that GET_THREAD_INFO works).

I will split off the THREAD_INFO() change.


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