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

[Andrew Cooper <amc96@xxxxxxxx>]

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

~Andrew