[PATCH] tools/libxenguest: Fix max_extd_leaf calculation for legacy restore

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

0x1c is lower than any value which will actually be observed in
p->extd.max_leaf, but higher than the logical 9 leaves worth of extended data
on Intel systems, causing x86_cpuid_copy_to_buffer() to fail with -ENOBUFS.

Correct the calculation.

The problem was first noticed in c/s 34990446ca9 "libxl: don't ignore the
return value from xc_cpuid_apply_policy" but introduced earlier.

Fixes: 34990446ca91 ("libxl: don't ignore the return value from 
xc_cpuid_apply_policy")
Reported-by: Olaf Hering <olaf@xxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Ian Jackson <iwj@xxxxxxxxxxxxxx>
CC: Olaf Hering <olaf@xxxxxxxxx>

Olaf - as I've changed the fix, I haven't included your T-by tag, but I'm
confident that this will suitably address the issue.
---
 tools/libs/guest/xg_cpuid_x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index e01d657e0394..0c9c4fefc1ef 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -513,7 +513,7 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
         /* Clamp maximum leaves to the ones supported on 4.12. */
         p->basic.max_leaf = min(p->basic.max_leaf, 0xdu);
         p->feat.max_subleaf = 0;
-        p->extd.max_leaf = min(p->extd.max_leaf, 0x1cu);
+        p->extd.max_leaf = min(p->extd.max_leaf, 0x8000001c);
     }
 
     if ( featureset )
-- 
2.11.0