[PATCH] x86/cpuid: prevent decreasing of hypervisor max leaf on migration

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

In order to be compatible with previous Xen versions, and not change
max hypervisor leaf as a result of a migration, keep the clamping of
the maximum leaf value provided to XEN_CPUID_MAX_NUM_LEAVES, instead
of doing it based on the domain type. Also set the default maximum
leaf without taking the domain type into account. The maximum
hypervisor leaf is not migrated, so we need the default to not regress
beyond what might already be reported to a guest by existing Xen
versions.

This is a partial revert of 540d911c28 and restores the previous
behaviour and assures that HVM guests won't see it's maximum
hypervisor leaf reduced from 5 to 4 as a result of a migration.

Fixes: 540d911c28 ('x86/CPUID: shrink max_{,sub}leaf fields according to actual 
leaf contents')
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Cc: Ian Jackson <iwj@xxxxxxxxxxxxxx>
---
Regarding release risks:

This is a partial revert of a commit.  The main issues could be that a
partial revert could break the build or leave the remaining code in a
non-working condition.

Breaking the build will be easily discovered by our automated testing,
while leaving the remaining code in a broken state is unlikely, as the
chunks reverted are isolated from the rest of the change in
540d911c28.
---
 xen/arch/x86/traps.c                | 6 ++----
 xen/include/public/arch-x86/cpuid.h | 6 +-----
 2 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index a1c2adb7ad..79fd276a41 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1086,15 +1086,13 @@ void cpuid_hypervisor_leaves(const struct vcpu *v, 
uint32_t leaf,
     uint32_t base = is_viridian_domain(d) ? 0x40000100 : 0x40000000;
     uint32_t idx  = leaf - base;
     unsigned int limit = is_viridian_domain(d) ? p->hv2_limit : p->hv_limit;
-    unsigned int dflt = is_pv_domain(d) ? XEN_CPUID_MAX_PV_NUM_LEAVES
-                                        : XEN_CPUID_MAX_HVM_NUM_LEAVES;
 
     if ( limit == 0 )
         /* Default number of leaves */
-        limit = dflt;
+        limit = XEN_CPUID_MAX_NUM_LEAVES;
     else
         /* Clamp toolstack value between 2 and MAX_NUM_LEAVES. */
-        limit = min(max(limit, 2u), dflt);
+        limit = min(max(limit, 2u), XEN_CPUID_MAX_NUM_LEAVES + 0u);
 
     if ( idx > limit )
         return;
diff --git a/xen/include/public/arch-x86/cpuid.h 
b/xen/include/public/arch-x86/cpuid.h
index 00926b1fef..ce46305bee 100644
--- a/xen/include/public/arch-x86/cpuid.h
+++ b/xen/include/public/arch-x86/cpuid.h
@@ -113,10 +113,6 @@
 /* Max. address width in bits taking memory hotplug into account. */
 #define XEN_CPUID_MACHINE_ADDRESS_WIDTH_MASK (0xffu << 0)
 
-#define XEN_CPUID_MAX_PV_NUM_LEAVES 5
-#define XEN_CPUID_MAX_HVM_NUM_LEAVES 4
-#define XEN_CPUID_MAX_NUM_LEAVES \
-    (XEN_CPUID_MAX_PV_NUM_LEAVES > XEN_CPUID_MAX_HVM_NUM_LEAVES ? \
-     XEN_CPUID_MAX_PV_NUM_LEAVES : XEN_CPUID_MAX_HVM_NUM_LEAVES)
+#define XEN_CPUID_MAX_NUM_LEAVES 5
 
 #endif /* __XEN_PUBLIC_ARCH_X86_CPUID_H__ */
-- 
2.33.0