Re: [PATCH 2/3] x86/spec-ctrl: Drop SPEC_CTRL_{ENTRY_FROM,EXIT_TO}_HVM

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.01.2022 12:41, Andrew Cooper wrote:
> On 17/01/2022 11:22, Jan Beulich wrote:
>> On 13.01.2022 17:38, Andrew Cooper wrote:
>>> --- a/xen/arch/x86/hvm/svm/entry.S
>>> +++ b/xen/arch/x86/hvm/svm/entry.S
>>> @@ -59,7 +59,7 @@ __UNLIKELY_END(nsvm_hap)
>>>          mov VCPUMSR_spec_ctrl_raw(%rax), %eax
>>>  
>>>          /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
>>> -        SPEC_CTRL_EXIT_TO_HVM   /* Req: a=spec_ctrl %rsp=regs/cpuinfo, 
>>> Clob: cd */
>>> +        /* SPEC_CTRL_EXIT_TO_SVM   (nothing currently) */
>>>  
>>>          pop  %r15
>>>          pop  %r14
>>> @@ -86,7 +86,8 @@ __UNLIKELY_END(nsvm_hap)
>>>  
>>>          GET_CURRENT(bx)
>>>  
>>> -        SPEC_CTRL_ENTRY_FROM_HVM    /* Req: b=curr %rsp=regs/cpuinfo, 
>>> Clob: acd */
>>> +        /* SPEC_CTRL_ENTRY_FROM_SVM    Req: b=curr %rsp=regs/cpuinfo, 
>>> Clob: ac  */
>>> +        ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
>> Just for my own understanding: The comments you add aren't commented
>> out macro invocations (as I did read it first), but comments naming
>> would-be-macros which are then expanded "manually"? That is partly
>> because initially I read the description saying "Expand and drop the
>> macros" as meaning that the macros grow in what they contain, which
>> looked contradictory to them getting dropped at the same time.
>> Perhaps me not sufficiently understanding the difference between all
>> possible meanings of "expand" vs "extend" ...
> 
> They're grep fodder to be able to easily locate where we're doing
> entry/exit speculation logic.

And I don't suppose I could talk you into replacing the use of "expand"
in the description, even if it was just me who was mislead? Perhaps by
"Open-code and drop the macros"?

>  These paths will continue to diverge over
> time, and cannot have a common implementation.

That's understood.

Jan