Re: [PATCH v2 4/4] x86/spec-ctrl: Fix NMI race condition with VT-x MSR_SPEC_CTRL handling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.01.2022 18:00, Andrew Cooper wrote:
> On 19/01/2022 13:42, Jan Beulich wrote:
>> On 17.01.2022 19:34, Andrew Cooper wrote:
>>> --- a/xen/arch/x86/hvm/vmx/entry.S
>>> +++ b/xen/arch/x86/hvm/vmx/entry.S
>>> @@ -35,7 +35,14 @@ ENTRY(vmx_asm_vmexit_handler)
>>>  
>>>          /* SPEC_CTRL_ENTRY_FROM_VMX    Req: b=curr %rsp=regs/cpuinfo, 
>>> Clob: acd */
>>>          ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
>>> -        ALTERNATIVE "", DO_SPEC_CTRL_ENTRY_FROM_HVM, X86_FEATURE_SC_MSR_HVM
>>> +
>>> +        .macro restore_spec_ctrl
>>> +            mov    $MSR_SPEC_CTRL, %ecx
>>> +            movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
>>> +            xor    %edx, %edx
>>> +            wrmsr
>>> +        .endm
>>> +        ALTERNATIVE "", restore_spec_ctrl, X86_FEATURE_SC_MSR_HVM
>>>          /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
>>>  
>>>          /* Hardware clears MSR_DEBUGCTL on VMExit.  Reinstate it if 
>>> debugging Xen. */
>>> @@ -82,8 +89,7 @@ UNLIKELY_END(realmode)
>>>          mov VCPUMSR_spec_ctrl_raw(%rax), %eax
>>>  
>>>          /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
>>> -        /* SPEC_CTRL_EXIT_TO_VMX   Req: a=spec_ctrl %rsp=regs/cpuinfo, 
>>> Clob: cd */
>>> -        ALTERNATIVE "", DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_HVM
>>> +        /* SPEC_CTRL_EXIT_TO_VMX   Req: %rsp=regs/cpuinfo              
>>> Clob:    */
>>>          ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)), 
>>> X86_FEATURE_SC_VERW_HVM
>> I notice you did update this clobber remark, but what about the one further
>> up in context?
> 
> What about it?  It still clobbers %eax, %ecx and %edx.

Oh, sorry - I did look at DO_OVERWRITE_RSB only, not paying attention
to the now open-coded 2nd part, which - due to the blank line - doesn't
appear connected to the comment anymore.

Jan