[Xen-devel] [PATCH] x86/cpuid: Fix handling of the CPUID.7[0].eax levelling MSR

[Andrew Cooper <andrew.cooper3@citrix.com>]

7a0 is an integer field, not a mask - taking the logical and of the hardware
and policy values results in nonsense.  Instead, take the policy value
directly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>

Even Rome hardware has 7[0].eax still as 0, and there is no sensible reason to
set max_subleaf higher at this point, so this is only a latent bug for now.
---
 xen/arch/x86/domctl.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index 1e98fc8009..35ad8cb51c 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -218,11 +218,16 @@ static int update_domain_cpuid_info(struct domain *d,
         if ( is_pv_domain(d) && ((levelling_caps & LCAP_7ab0) == LCAP_7ab0) )
         {
             uint64_t mask = cpuidmask_defaults._7ab0;
-            uint32_t eax = ctl->eax;
-            uint32_t ebx = p->feat._7b0;
 
-            if ( boot_cpu_data.x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON) )
-                mask &= ((uint64_t)eax << 32) | ebx;
+            /*
+             * Leaf 7[0].eax is max_subleaf, not a feature mask.  Take it
+             * wholesale from the policy, but clamp the features in 7[0].ebx
+             * per usual.
+             */
+            if ( boot_cpu_data.x86_vendor &
+                 (X86_VENDOR_AMD | X86_VENDOR_HYGON) )
+                mask = (((uint64_t)p->feat.max_subleaf << 32) |
+                        ((uint32_t)mask | p->feat._7b0));
 
             d->arch.pv.cpuidmasks->_7ab0 = mask;
         }
-- 
2.11.0


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