Re: [Xen-devel] [PATCH v9 06/11] x86/entry: Organise the use of MSR_SPEC_CTRL at each entry/exit point

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/01/2018 22:27, Boris Ostrovsky wrote:
> On 01/19/2018 08:36 AM, Andrew Cooper wrote:
>> On 19/01/18 11:43, Jan Beulich wrote:
>>
>>>> @@ -99,6 +106,10 @@ UNLIKELY_END(realmode)
>>>>  .Lvmx_vmentry_fail:
>>>>          sti
>>>>          SAVE_ALL
>>>> +
>>>> +        SPEC_CTRL_ENTRY_FROM_PV /* Req: %rsp=regs/cpuinfo Clob: acd */
>>> I think the use of the PV variant here requires a comment.
>> Oh.  It used to have one...  I'll try to find it.
> I, in fact, meant to ask about this for a long time and always forgot.
> Perhaps your comment will say more than just why a PV variant is used
> here but in case it won't --- why do we have *any* mitigation here? We
> are never returning to the guest, do we?

We never return to *this* guest, but we are still open to abuse from a
separate hyperthread, so still need to set SPEC_CTRL.IBRS if we are
using IBRS for safety.  (If we are using lfence+jmp or repoline then we
don't need this change, but its not a hotpath so doesn't warrant yet
another variant of SPEC_CTRL_ENTRY_FROM_*.)

~Andrew

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