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

[Andrew Cooper <amc96@xxxxxxxx>]

On 14/01/2022 11:42, Roger Pau Monné wrote:
> On Thu, Jan 13, 2022 at 04:38:32PM +0000, Andrew Cooper wrote:
>> These were written before Spectre/Meltdown went public, and there was large
>> uncertainty in how the protections would evolve.  As it turns out, they're
>> very specific to Intel hardware, and not very suitable for AMD.
>>
>> Expand and drop the macros.  No change at all for VT-x.
>>
>> For AMD, the only relevant piece of functionality is DO_OVERWRITE_RSB,
>> although we will soon be adding (different) logic to handle MSR_SPEC_CTRL.
>>
>> This has a marginal improvement of removing an unconditional pile of 
>> long-nops
>> from the vmentry/exit path.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>
> I wonder however if it would be clearer to define
> SPEC_CTRL_ENTRY_FROM_{SVM,VMX} and EXIT macros in spec_ctrl_asm.h
> (even if just used in a single place) so that all the related SPEC
> macros are in a single file.

For AMD MSR_SPEC_CTRL support, I'm going to need to shift the STGI/CLGI,
then call up into C, and I do not thing this is appropriate to have
separated out into spec_ctrl_asm.h

I left the comments intact deliberately so `grep SPEC_CTRL_ENTRY` still
lets you find everything.

The main difference between VT-x/SVM and PV is that for HVM, we have
this dance exactly once.  For PV, it is repeated multiple times in
subtly different ways, which is why having all the spec ctrl shadowing
logic together makes sense.

Its pretty ugly whichever way you look at it.

~Andrew