[Xen-devel] [PATCH] tools/libxc: Fix the reported max_leaf values for PV guests

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

When iterating through CPUID leaves to generating a policy, libxc will clip
itself at the hardcoded maxima, meaning that no data outside of the hardcoded
maxima are provided to Xen (in turn, causing Xen to return zeros if these
leaves are requested.)

The HVM code also clips the max_leaf data reported to the guest, but the PV
side didn't.

This results in a PV guest using the emulated CPUID, or via Xen using CPUID
faulting, to observe a max_leaf higher than the toolstack wants, although with
zeros being returned in the intervening leaves.

Fix the PV side to behave like the HVM side, and clip the max_leaf values in
leaf 0 and 0x80000000.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>

This should be backported to stable trees.

Discovered during my hypervisor-side work which does properly clip the
max_leaf values reported, at which point my diff against the guest-observable
CPUID information at the beginning of the series suddenly became non-zero.
---
 tools/libxc/xc_cpuid_x86.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
index e9e3691..b32001b3 100644
--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -615,6 +615,11 @@ static void xc_cpuid_pv_policy(xc_interface *xch,
 {
     switch ( input[0] )
     {
+    case 0x00000000:
+        if ( regs[0] > DEF_MAX_BASE )
+            regs[0] = DEF_MAX_BASE;
+        break;
+
     case 0x00000001:
     {
         /* Host topology exposed to PV guest.  Provide host value. */
@@ -655,6 +660,16 @@ static void xc_cpuid_pv_policy(xc_interface *xch,
         xc_cpuid_config_xsave(xch, info, input, regs);
         break;
 
+    case 0x80000000:
+    {
+        unsigned int max = info->vendor == VENDOR_AMD
+            ? DEF_MAX_AMDEXT : DEF_MAX_INTELEXT;
+
+        if ( regs[0] > max )
+            regs[0] = max;
+        break;
+    }
+
     case 0x80000001:
     {
         /* Host topology exposed to PV guest.  Provide host CMP_LEGACY value. 
*/
-- 
2.1.4


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