Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL_MSG

[Josh Poimboeuf]

On Tue, Oct 07, 2025 at 04:08:21PM -0700, Pawan Gupta wrote:
> On Wed, Oct 08, 2025 at 12:12:29AM +0200, Borislav Petkov wrote:
> > On Tue, Oct 07, 2025 at 11:22:57AM -0700, Pawan Gupta wrote:
> > > IMO, rather than targeting the mitigation enabling code it might make more
> > > sense to compile out the actual mitigations scattered accross the kernel.
> > > This may also improve performance by reducing the code footprint, and can
> > > also help getting a cleaner disassembly.

Isn't that what CONFIG_CPU_MITIGATIONS=n already does today?

> > Probably... however, it needs to be done smartly because sprinkling ifdeffery
> > and turning what is an already unreadable mess into a bigger abomination,
> > won't fly. Perhaps split out the mitigations glue into separate compilation
> > units and build-disable them... we'll see.
> 
> Ya, that would be better.

On a sort of related note, it's confusing that there are two completely
different classes of MITIGATION options which get conflated:

1) compile the kernel with support for certain mitigations:

    MITIGATION_RETPOLINE
    MITIGATION_RETHUNK
    MITIGATION_PAGE_TABLE_ISOLATION
    MITIGATION_UNRET_ENTRY
    MITIGATION_CALL_DEPTH_TRACKING
    MITIGATION_IBPB_ENTRY
    MITIGATION_IBRS_ENTRY
    MITIGATION_SRSO
    MITIGATION_SLS

2) enable bug-specific runtime defaults:

    MITIGATION_GDS
    MITIGATION_RFDS
    MITIGATION_SPECTRE_BHI
    MITIGATION_MDS
    MITIGATION_TAA
    MITIGATION_MMIO_STALE_DATA
    MITIGATION_L1TF
    MITIGATION_RETBLEED
    MITIGATION_SPECTRE_V1
    MITIGATION_SPECTRE_V2
    MITIGATION_SRBDS
    MITIGATION_SSB
    MITIGATION_ITS
    MITIGATION_TSA
    MITIGATION_VMSCAPE

In general, #1 uglify the kernel.  And some #2 depend on #1.

IMO, we should rename the #1 options.  For example:

  MITIGATION_RETPOLINE -> X86_UGLY_INDIRECT_THUNKS
  MITIGATION_RETHUNK   -> X86_UGLY_RETURN_THUNKS

  etc...

Then one only needs to grep their .config file for UGLY to understand
why their disassembly is so inscrutable ;-)

-- 
Josh