x86/cpuid: Fix handling of XSAVE dynamic leaves

[ This is a minimal backport of commit 71cacfb035f4 ("x86/cpuid: Fix handling
  of XSAVE dynamic leaves") to fix the bugs without depending on the large
  rework of XSTATE handling in Xen 4.19 ]

First, if XSAVE is available in hardware but not visible to the guest, the
dynamic leaves shouldn't be filled in.

Second, the comment concerning XSS state is wrong.  VT-x doesn't manage
host/guest state automatically, but there is provision for "host only" bits to
be set, so the implications are still accurate.

In Xen 4.18, no XSS states are supported, so it's safe to keep deferring to
real hardware.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 71cacfb035f4a78ee10970dc38a3baa04d387451
master date: 2024-06-19 13:00:06 +0100

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index f311372cdf1f779190c806a27bec594b8081efd1..ead6dde3f2f5feb675586da7e73a7e744287bc41 100644 (file)
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -330,24 +330,20 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
     case XSTATE_CPUID:
         switch ( subleaf )
         {
-        case 1:
-            if ( p->xstate.xsavec || p->xstate.xsaves )
-            {
-                /*
-                 * TODO: Figure out what to do for XSS state.  VT-x manages
-                 * host vs guest MSR_XSS automatically, so as soon as we start
-                 * supporting any XSS states, the wrong XSS will be in
-                 * context.
-                 */
-                BUILD_BUG_ON(XSTATE_XSAVES_ONLY != 0);
-
-                /*
-                 * Read CPUID[0xD,0/1].EBX from hardware.  They vary with
-                 * enabled XSTATE, and appropraite XCR0|XSS are in context.
-                 */
+            /*
+             * Read CPUID[0xd,0/1].EBX from hardware.  They vary with enabled
+             * XSTATE, and the appropriate XCR0 is in context.
+             */
         case 0:
-                res->b = cpuid_count_ebx(leaf, subleaf);
-            }
+            if ( p->basic.xsave )
+                res->b = cpuid_count_ebx(0xd, 0);
+            break;
+
+        case 1:
+            /* This only works because Xen doesn't support XSS states yet. */
+            BUILD_BUG_ON(XSTATE_XSAVES_ONLY != 0);
+            if ( p->xstate.xsavec )
+                res->b = cpuid_count_ebx(0xd, 1);
             break;
         }
         break;