From 57f26ae72983095d0258e391041dfb8864f769e5 Mon Sep 17 00:00:00 2001 From: Eduardo Habkost Date: Thu, 20 Dec 2012 16:43:48 -0200 Subject: [PATCH] target-i386: CPUID: return highest basic leaf if eax > cpuid_xlevel MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit This fixes a subtle bug. A bug that probably won't cause trouble for any existing OS, but a bug anyway: Intel SDM Volume 2, CPUID Instruction states: > Two types of information are returned: basic and extended function > information. If a value entered for CPUID.EAX is higher than the maximum > input value for basic or extended function for that processor then the > data for the highest basic information leaf is returned. For example, > using the Intel Core i7 processor, the following is true: > > CPUID.EAX = 05H (* Returns MONITOR/MWAIT leaf. *) > CPUID.EAX = 0AH (* Returns Architectural Performance Monitoring leaf. *) > CPUID.EAX = 0BH (* Returns Extended Topology Enumeration leaf. *) > CPUID.EAX = 0CH (* INVALID: Returns the same information as CPUID.EAX = 0BH. *) > CPUID.EAX = 80000008H (* Returns linear/physical address size data. *) > CPUID.EAX = 8000000AH (* INVALID: Returns same information as CPUID.EAX = 0BH. *) AMD's CPUID Specification, on the other hand, is less specific: > The CPUID instruction supports two sets or ranges of functions, > standard and extended. > > • The smallest function number of the standard function range is > Fn0000_0000. The largest function num- ber of the standard function > range, for a particular implementation, is returned in CPUID > Fn0000_0000_EAX. > > • The smallest function number of the extended function range is > Fn8000_0000. The largest function num- ber of the extended function > range, for a particular implementation, is returned in CPUID > Fn8000_0000_EAX. > > Functions that are neither standard nor extended are undefined and > should not be relied upon. QEMU's behavior matched Intel's specification before, but this was changed by commit b3baa152aaef1905876670590275c2dd0bbb088c. This patch restores the behavior documented by Intel when cpuid_xlevel2 is 0. The existing behavior when cpuid_xlevel2 is set (falling back to level=cpuid_xlevel) is being kept, as I couldn't find any public documentation on the CPUID 0xC0000000 function range on Centaur CPUs. Signed-off-by: Eduardo Habkost Signed-off-by: Anthony Liguori --- target-i386/cpu.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/target-i386/cpu.c b/target-i386/cpu.c index 1837f5af04..3cd1cee540 100644 --- a/target-i386/cpu.c +++ b/target-i386/cpu.c @@ -1648,7 +1648,11 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count, index = env->cpuid_xlevel; } } else { - index = env->cpuid_xlevel; + /* Intel documentation states that invalid EAX input will + * return the same information as EAX=cpuid_level + * (Intel SDM Vol. 2A - Instruction Set Reference - CPUID) + */ + index = env->cpuid_level; } } } else { -- 2.11.4.GIT