Re: [PATCH 17/24] x86/entry: Introduce struct ist_regs

[Lai Jiangshan <laijs@linux.alibaba.com>]

On 2021/9/10 12:31, H. Peter Anvin wrote:
> Note: the examples in this email all compiled with:
> 
> gcc -O2 -mpreferred-stack-boundary=3 -mcmodel=kernel
> 
> The disassembly has been slightly simplified.

Saving results in registers is non-flexible no matter in ASM or hack in C like this.

Saving CR3 in ist_regs is not different than saving rax in pt_regs, and both of
ist_regs and embedded pt_regs can be moved when stack is required to switch.

I prefer to my original design.

> 
> 
> On 9/9/21 5:18 PM, Lai Jiangshan wrote:
>>
>> This patch was over designed.
>>
>> In ASM code, we can easily save results in the callee-saved registers.
>> For example, rc3 is saved in %r14, gsbase info is saved in %rbx.
>>
>> And in C code, we can't save results in registers.  And I thought there was
>> no place to save the results because the CR3 and gsbase are not kernel's.
>> So I extended the pt_regs to ist_regs to save the results.
>>
>> But it was incorrect.  The results can be saved in percpu data at the end of
>> paranoid_entry() after the CR3/gsbase are settled down.  And the results
>> can be read at the beginning of paranoid_exit() before the CR3/gsbase are
>> switched to the interrupted context's.
>>
> 
> OK, count me confused. Of course you can save results in caller-saved registers in C; it is kind of what they do:
> 
> 
> extern void bar(void);
> 
> unsigned long foo(unsigned long v)
> {
>      bar();
>      return v;
> }
> 
> 0000000000000000 <foo>:
>     0:   41 54                   push   %r12
>     2:   49 89 fc                mov    %rdi,%r12
>     5:   e8 00 00 00 00          callq  bar
>     a:   4c 89 e0                mov    %r12,%rax
>     d:   41 5c                   pop    %r12
>     f:   c3                      retq
> 
> Now, if you need to specify *which* registers, you have to declare them as global register variables - NOT local (which 
> have completely different semantics). This also means that you (probably) want to isolate this code into its own 
> compilation unit, because it will prevent any other code in the same .c file from using that register as well.
> 
> For example:
> 
> register unsigned long r14 asm("%r14");
> unsigned long foo(unsigned long v)
> {
>      r14 = v;
>      bar();
>      v = r14;
>      return v;
> }
> 
> 0000000000000000 <foo>:
>     0:   49 89 fe                mov    %rdi,%r14
>     3:   e8 00 00 00 00          callq  bar
>     8:   4c 89 f0                mov    %r14,%rax
>     b:   c3                      retq
> 
> WARNING: This also means that gcc will happily discard the old value in %r14, so if you need it preserved you have to do 
> so explicitly; if you are called direct from assembly and are happy to lose the value then the above code is fine -- and 
> it is even slightly more efficient!
> 
> For preserving the old r14, in this case:
> 
> register unsigned long r14 asm("%r14");
> unsigned long foo(unsigned long v)
> {
>      unsigned long saved_r14 = r14;
>      r14 = v;
>      bar();
>      v = r14;
>      r14 = saved_r14;
>      return v;
> }
> 
> 0000000000000000 <foo>:
>     0:   53                      push   %rbx
>     1:   4c 89 f3                mov    %r14,%rbx
>     4:   49 89 fe                mov    %rdi,%r14
>     7:   e8 00 00 00 00          callq  bar
>     c:   4c 89 f0                mov    %r14,%rax
>     f:   49 89 de                mov    %rbx,%r14
>    12:   5b                      pop    %rbx
>    13:   c3                      retq
> 
> 
> HOWEVER, if you are relying on not using the stack, then using C code is probably very much not a good idea. It is very 
> hard to guarantee that just because the C compiler is *currently* not using a stack, that it won't do so *in the future*.
> 
> Again, finally, local register variables DO NOT WORK, this does NOT do what you expect:
> 
> unsigned long foo(unsigned long v)
> {
>      register unsigned long r14 asm("%r14") = v;
>      bar();
>      return r14;
> }
> 
> 0000000000000000 <foo>:
>     0:   41 54                   push   %r12
>     2:   49 89 fc                mov    %rdi,%r12
>     5:   e8 00 00 00 00          callq  bar
>     a:   4c 89 e0                mov    %r12,%rax
>     d:   41 5c                   pop    %r12
>     f:   c3                      retq
> 
>