[PATCH 1/5] target/i386: Add support for LASS in CPUID enumeration

[Kishen Maloor <kishen.maloor@intel.com>]

From: Isaku Yamahata <isaku.yamahata@intel.com>

Linear Address Space Separation (LASS) is a security feature that
prevents a class of side-channel attacks relying on speculative
accesses across the user/kernel boundary. Paging, along with SMEP
and SMAP, already provides mode-based access protection, but
enforcing it requires a page walk whose timing can leak the
layout of kernel memory.

LASS applies the equivalent protections during linear-address
pre-processing, before any page walk. Given the usual partitioning of
the linear address space into a user half (bit 63 clear) and a
supervisor half (bit 63 set), an access targeting the opposite half is
rejected on the basis of bit 63 alone, raising a #GP. LASS is enabled
via CR4.LASS[bit 27] and applies only in IA-32e mode.

Feature bit:
  CPUID.(EAX=7,ECX=1):EAX[6]

A CPUID_7_1_EAX_LASS macro was previously added in commit 31df29c532a9
("i386/tdx: Add supported CPUID bits related to TD Attributes"), but the
bit was left unnamed in feature_word_info[FEAT_7_1_EAX]. Add the "lass"
feature name to expose it via -cpu host, -cpu max, or an explicit +lass.

Exposing LASS to a guest also requires KVM support: KVM must validate
the CPUID bit and CR4.LASS, and enforce LASS violations in its
instruction emulator.

LASS is not implemented in TCG, so the bit is not added to
TCG_7_1_EAX_FEATURES.

More details can be found in the Intel 64 and IA-32 Architectures
Software Developer's Manual, Volume 3A, Section 4.3,
"Linear-Address-Space Separation (LASS)".

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
[kishen: rewrote commit message, rebased]
Signed-off-by: Kishen Maloor <kishen.maloor@intel.com>
---
KVM support for LASS is currently under review:
https://lore.kernel.org/kvm/20260806011536.4172258-1-sohil.mehta@intel.com/

 target/i386/cpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 5805d33ab9..5496e9475b 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -1243,7 +1243,7 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
         .type = CPUID_FEATURE_WORD,
         .feat_names = {
             "sha512", "sm3", "sm4", NULL,
-            "avx-vnni", "avx512-bf16", NULL, "cmpccxadd",
+            "avx-vnni", "avx512-bf16", "lass", "cmpccxadd",
             NULL, NULL, "fzrm", "fsrs",
             "fsrc", NULL, NULL, NULL,
             NULL, "fred", "lkgs", "wrmsrns",
-- 
2.47.1