Re: [PATCH v5 2/8] x86/bugs: Allow spectre_v2=ibrs on x86 vendors other than Intel

On Wed, Aug 26, 2026 at 05:35:04PM -0500, Kim Phillips wrote:

Prepare for legacy IBRS toggling on AMD, where the BTB Isolation
SEV-SNP feature uses it to optimize the VM exit-to-re-entry path.
Commit 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to
support Kernel IBRS") restricted the option to Intel because that was the
only vendor that needed it at the time; nothing about the mechanism
is Intel-specific.

Keep the IBRS-trumps-retbleed logic in retbleed_update_mitigation()
Intel-only.  Legacy SPEC_CTRL.IBRS does not mitigate AMD's Branch
Type Confusion RETBleed variant (RET prediction uses the Return
Address Predictor, not the indirect branch predictors IBRS
restricts), so letting SPECTRE_V2_IBRS trump retbleed on AMD would
silently drop the UNRET/IBPB mitigation that does cover it.

On AMD the decoupling is total: retbleed mitigation selection never
consults spectre_v2=, so spectre_v2=ibrs neither adds nor removes
RETBleed coverage.  A kernel built without MITIGATION_UNRET_ENTRY and
MITIGATION_IBPB_ENTRY already reports RETBleed as "Vulnerable" via the
retbleed sysfs node and boot log regardless of the spectre_v2= value,
so there is no silent gap in the spectre_v2=ibrs path to warn about --
and a warning there would wrongly imply the Intel-style IBRS/RETBleed
coupling exists on AMD.

Also drop CPU_SUP_INTEL from CONFIG_MITIGATION_IBRS_ENTRY's depends
line: the IBRS_ENTER/IBRS_EXIT macros are vendor-neutral, and the
Intel-only restriction would silently redirect spectre_v2=ibrs to
AUTO on AMD-only kernels.

In spectre_v2_apply_mitigation(), route AutoIBRS-capable CPUs to
EFER.AUTOIBRS only for the eIBRS modes.  Previously any IBRS mode used
EFER.AUTOIBRS when the CPU had AutoIBRS, which was unreachable while
spectre_v2=ibrs was Intel-only, but would now hand spectre_v2=ibrs the
always-on AutoIBRS behaviour instead of the toggleable SPEC_CTRL.IBRS
the option asks for.

Finally, clear EFER.AUTOIBRS at the top of cpu_select_mitigations(),
alongside the existing SPEC_CTRL kexec cleanup.  head_64.S preserves
incoming EFER bits, so a kexec from a kernel that ran in AutoIBRS mode
carries the bit into the new kernel; without an explicit clear the CPU
stays in AutoIBRS mode while sysfs reports e.g. "Mitigation: IBRS" or a
retpoline mode, diverging from the actual hardware state.  On a normal
cold boot the bit is already clear, so the msr_clear_bit() is a no-op
there.  Clearing on the boot CPU suffices for APs, since it precedes the
init_real_mode() EFER snapshot used by the AP trampoline.

Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Kim Phillips <redacted>

Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>