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.

Introduce xstate_compressed_size() to mirror the uncompressed one.  Cross
check it at boot.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Oleksii Kurochko <oleksii.kurochko@gmail.com>

v3:
 * Adjust commit message about !XSAVE guests
 * Rebase over boot time cross check
 * Use raw policy
v4:
 * Drop the TODO comment.  The CPUID path is always liable to pass 0 here.
 * ASSERT() a nonzero c->size like we do in the uncompressed helper.
---
 xen/arch/x86/cpuid.c              | 24 +++++++--------------
 xen/arch/x86/include/asm/xstate.h |  1 +
 xen/arch/x86/xstate.c             | 36 +++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 7a38e032146a..a822e80c7ea7 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -330,23 +330,15 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
     case XSTATE_CPUID:
         switch ( subleaf )
         {
-        case 1:
-            if ( !p->xstate.xsavec && !p->xstate.xsaves )
-                break;
-
-            /*
-             * 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);
-            fallthrough;
         case 0:
-            /*
-             * Read CPUID[0xD,0/1].EBX from hardware.  They vary with enabled
-             * XSTATE, and appropriate XCR0|XSS are in context.
-             */
-            res->b = cpuid_count_ebx(leaf, subleaf);
+            if ( p->basic.xsave )
+                res->b = xstate_uncompressed_size(v->arch.xcr0);
+            break;
+
+        case 1:
+            if ( p->xstate.xsavec )
+                res->b = xstate_compressed_size(v->arch.xcr0 |
+                                                v->arch.msrs->xss.raw);
             break;
         }
         break;
diff --git a/xen/arch/x86/include/asm/xstate.h b/xen/arch/x86/include/asm/xstate.h
index bfb66dd766b6..da1d89d2f416 100644
--- a/xen/arch/x86/include/asm/xstate.h
+++ b/xen/arch/x86/include/asm/xstate.h
@@ -109,6 +109,7 @@ void xstate_free_save_area(struct vcpu *v);
 int xstate_alloc_save_area(struct vcpu *v);
 void xstate_init(struct cpuinfo_x86 *c);
 unsigned int xstate_uncompressed_size(uint64_t xcr0);
+unsigned int xstate_compressed_size(uint64_t xstates);
 
 static inline uint64_t xgetbv(unsigned int index)
 {
diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 8edc4792a8fd..31bf2dc95f57 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -619,6 +619,36 @@ unsigned int xstate_uncompressed_size(uint64_t xcr0)
     return size;
 }
 
+unsigned int xstate_compressed_size(uint64_t xstates)
+{
+    unsigned int i, size = XSTATE_AREA_MIN_SIZE;
+
+    if ( xstates == 0 )
+        return 0;
+
+    if ( xstates <= (X86_XCR0_SSE | X86_XCR0_FP) )
+        return size;
+
+    /*
+     * For the compressed size, every non-legacy component matters.  Some
+     * componenets require aligning to 64 first.
+     */
+    xstates &= ~(X86_XCR0_SSE | X86_XCR0_FP);
+    for_each_set_bit ( i, &xstates, 63 )
+    {
+        const struct xstate_component *c = &raw_cpu_policy.xstate.comp[i];
+
+        ASSERT(c->size);
+
+        if ( c->align )
+            size = ROUNDUP(size, 64);
+
+        size += c->size;
+    }
+
+    return size;
+}
+
 struct xcheck_state {
     uint64_t states;
     uint32_t uncomp_size;
@@ -681,6 +711,12 @@ static void __init check_new_xstate(struct xcheck_state *s, uint64_t new)
                   s->states, &new, hw_size, s->comp_size);
 
         s->comp_size = hw_size;
+
+        xen_size = xstate_compressed_size(s->states);
+
+        if ( xen_size != hw_size )
+            panic("XSTATE 0x%016"PRIx64", compressed hw size %#x != xen size %#x\n",
+                  s->states, hw_size, xen_size);
     }
     else if ( hw_size ) /* Compressed size reported, but no XSAVEC ? */
     {
-- 
2.39.2