[xen staging-4.17] x86/cpuid: Fix handling of XSAVE dynamic leaves

[patchbot@xxxxxxx]

commit 02f94eab2183202180689eb85a2408d21dbe5505
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Jun 26 14:13:37 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jun 26 14:13:37 2024 +0200

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 71cacfb035f4a78ee10970dc38a3baa04d387451
    master date: 2024-06-19 13:00:06 +0100
---
 xen/arch/x86/cpuid.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index f311372cdf..ead6dde3f2 100644
--- 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;
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.17