[xen master] x86/cpuid: prevent shrinking migrated policies max leaves

[patchbot@xxxxxxx]

commit 81da2b544cbb003a5447c9b14d275746ad22ab37
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Wed Nov 10 18:40:59 2021 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Nov 12 13:57:04 2021 +0000

    x86/cpuid: prevent shrinking migrated policies max leaves
    
    CPUID policies from guest being migrated shouldn't have the maximum
    leaves shrink, as that would be a guest visible change. The hypervisor
    has no knowledge on whether a guest has been migrated or is build from
    scratch, and hence it must not blindly shrink the CPUID policy in
    recalculate_cpuid_policy. Remove the
    x86_cpuid_policy_shrink_max_leaves call from recalculate_cpuid_policy.
    Removing such call could be seen as a partial revert of 540d911c28.
    
    Instead let the toolstack shrink the policies for newly created
    guests, while keeping the previous values for guests that are migrated
    in. Note that guests migrated in without a CPUID policy won't get any
    kind of shrinking applied.
    
    Fixes: 540d911c28 ('x86/CPUID: shrink max_{,sub}leaf fields according to 
actual leaf contents')
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>
---
 tools/libs/guest/xg_cpuid_x86.c | 7 +++++++
 xen/arch/x86/cpuid.c            | 2 --
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 198892ebdf..3ffd5f683b 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -638,6 +638,13 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
         }
     }
 
+    /*
+     * Do not try to shrink the policy if restoring, as that could cause
+     * guest visible changes in the maximum leaf fields.
+     */
+    if ( !restore )
+        x86_cpuid_policy_shrink_max_leaves(p);
+
     rc = x86_cpuid_copy_to_buffer(p, leaves, &nr_leaves);
     if ( rc )
     {
diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 2079a30ae4..8ac55f0806 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -719,8 +719,6 @@ void recalculate_cpuid_policy(struct domain *d)
 
     if ( !p->extd.page1gb )
         p->extd.raw[0x19] = EMPTY_LEAF;
-
-    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 int init_domain_cpuid_policy(struct domain *d)
--
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