[PATCH] x86/boot: Further simplify CR4 handling in dom0_construct_pv()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The logic would be more robust disabling SMAP based on its precense in CR4,
rather than on certain features.

A forthcoming feature, LASS, needs the same treatment here.  Introduce minimum
enumeration information, although it will take a bit more work to get LASS
fully usable in guests.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

I know LASS can't be used with traditional PV guests, but I have some PV-lite
plans.  The problem is the PV kernel, in CPL3, accessing addresses in the high
canonincal half.
---
 xen/arch/x86/include/asm/x86-defns.h        |  1 +
 xen/arch/x86/pv/dom0_build.c                | 18 ++++++++++--------
 xen/include/public/arch-x86/cpufeatureset.h |  1 +
 3 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/include/asm/x86-defns.h 
b/xen/arch/x86/include/asm/x86-defns.h
index caa92829eaa9..8f97fb1e6a12 100644
--- a/xen/arch/x86/include/asm/x86-defns.h
+++ b/xen/arch/x86/include/asm/x86-defns.h
@@ -75,6 +75,7 @@
 #define X86_CR4_PKE        0x00400000 /* enable PKE */
 #define X86_CR4_CET        0x00800000 /* Control-flow Enforcement Technology */
 #define X86_CR4_PKS        0x01000000 /* Protection Key Supervisor */
+#define X86_CR4_LASS       0x08000000 /* Linear Address Space Separation */
 
 /*
  * XSTATE component flags in XCR0 | MSR_XSS
diff --git a/xen/arch/x86/pv/dom0_build.c b/xen/arch/x86/pv/dom0_build.c
index 262edb6bf2f0..f5c868df384f 100644
--- a/xen/arch/x86/pv/dom0_build.c
+++ b/xen/arch/x86/pv/dom0_build.c
@@ -1057,29 +1057,31 @@ int __init dom0_construct_pv(struct domain *d,
                              module_t *initrd,
                              const char *cmdline)
 {
+    unsigned long cr4 = read_cr4();
+    unsigned long mask = X86_CR4_SMAP | X86_CR4_LASS;
     int rc;
 
     /*
-     * Clear SMAP in CR4 to allow user-accesses in construct_dom0().  This
-     * prevents us needing to write construct_dom0() in terms of
+     * Clear SMAP/LASS in CR4 to allow user-accesses in construct_dom0().
+     * This prevents us needing to write construct_dom0() in terms of
      * copy_{to,from}_user().
      */
-    if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
+    if ( cr4 & mask )
     {
         if ( IS_ENABLED(CONFIG_PV32) )
-            cr4_pv32_mask &= ~X86_CR4_SMAP;
+            cr4_pv32_mask &= ~mask;
 
-        write_cr4(read_cr4() & ~X86_CR4_SMAP);
+        write_cr4(cr4 & ~mask);
     }
 
     rc = dom0_construct(d, image, image_headroom, initrd, cmdline);
 
-    if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
+    if ( cr4 & mask )
     {
-        write_cr4(read_cr4() | X86_CR4_SMAP);
+        write_cr4(cr4);
 
         if ( IS_ENABLED(CONFIG_PV32) )
-            cr4_pv32_mask |= X86_CR4_SMAP;
+            cr4_pv32_mask |= mask;
     }
 
     return rc;
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index 8fa3fb711a8d..cbc0a3a8aa2b 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -303,6 +303,7 @@ XEN_CPUFEATURE(SM3,          10*32+ 1) /*A  SM3 
Instructions */
 XEN_CPUFEATURE(SM4,          10*32+ 2) /*A  SM4 Instructions */
 XEN_CPUFEATURE(AVX_VNNI,     10*32+ 4) /*A  AVX-VNNI Instructions */
 XEN_CPUFEATURE(AVX512_BF16,  10*32+ 5) /*A  AVX512 BFloat16 Instructions */
+XEN_CPUFEATURE(LASS,         10*32+ 6) /*   Linear Address Space Separation */
 XEN_CPUFEATURE(CMPCCXADD,    10*32+ 7) /*a  CMPccXADD Instructions */
 XEN_CPUFEATURE(FZRM,         10*32+10) /*A  Fast Zero-length REP MOVSB */
 XEN_CPUFEATURE(FSRS,         10*32+11) /*A  Fast Short REP STOSB */
-- 
2.39.5