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SubjectRe: KVM guest-kernel panics double fault
Hello, it is me again...

I improved the patch, so it is now working on bases of guest's pretends.
(For AMD and INTEL CPUs)
If anybody needs it, I can also publish my test-VM - just ask me...

On 12/29/11 11:04, Avi Kivity wrote:
> The PROT32 check should be qualifed by a checking the the guest cpuid
> vendor is not AMD. Otherwise a guest that was migrated from an AMD host
> to an Intel host (this is what this emulation was written for in the
> first place) will #UD unexpectedly.
>
There are strange AMDs out there, where also again legacy-mode
syscalls are not available - or even syscall at all is not available.
The implemented (according to doku) tests should now cover all
cases.

I also think I ovserved some strange behaviour according to cpuid after
migration (change from Intel to AMD - see picture:
http://matrixstorm.com/software/linux/kvm/20111229/cpuidmagic.jpg )
But it is possible it is "only" an qemu-kvm (0.14.1-r2) bug ...

The patch I wrote is against 3.2, but I always tested with linux 3.1.7.
Hopfully it solve the problems to your satisfaction this time.


regards Stephan
From 904c45acea67d5b18796a2d97bce54097bd440fe Mon Sep 17 00:00:00 2001
From: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
Date: Sat, 7 Jan 2012 20:20:43 +0000
Subject: [PATCH 1/2] KVM: move "emul_to_vcpu"-makro to headerfile

In order to take advantage of "emul_to_vpu" in other
C-files, this makro has been moved to the headerfile.

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
---
arch/x86/kvm/x86.c | 3 ---
arch/x86/kvm/x86.h | 3 +++
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4c938da..ac949ba 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -64,9 +64,6 @@
#define KVM_MAX_MCE_BANKS 32
#define KVM_MCE_CAP_SUPPORTED (MCG_CTL_P | MCG_SER_P)

-#define emul_to_vcpu(ctxt) \
- container_of(ctxt, struct kvm_vcpu, arch.emulate_ctxt)
-
/* EFER defaults:
* - enable syscall per default because its emulated by KVM
* - enable LME and LMA per default on 64 bit KVM
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index d36fe23..644c843 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -4,6 +4,9 @@
#include <linux/kvm_host.h>
#include "kvm_cache_regs.h"

+#define emul_to_vcpu(ctxt) \
+ container_of(ctxt, struct kvm_vcpu, arch.emulate_ctxt)
+
static inline void kvm_clear_exception_queue(struct kvm_vcpu *vcpu)
{
vcpu->arch.exception.pending = false;
--
1.7.3.4
From 027d609f885c1f7d46afc0f68eab33c006fa57c6 Mon Sep 17 00:00:00 2001
From: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
Date: Sun, 8 Jan 2012 02:03:47 +0000
Subject: [PATCH 2/2] KVM: fix missing "illegal instruction"-trap in protected modes

On hosts without this patch, 32bit guests will crash
(and 64bit guests may behave in a wrong way) for
example by simply executing following nasm-demo-application:

[bits 32]
global _start
SECTION .text
_start: syscall

(I tested it with winxp and linux - both always crashed)

Disassembly of section .text:

00000000 <_start>:
0: 0f 05 syscall

The reason seems a missing "invalid opcode"-trap (int6) for the
syscall opcode "0f05", which is not available on Intel CPUs
within non-longmodes, as also on some AMD CPUs within legacy-mode.
(depending on CPU vendor, MSR_EFER and cpuid)

Because previous mentioned OSs may not engage corresponding
syscall target-registers (STAR, LSTAR, CSTAR), they remain
NULL and (non trapping) syscalls are leading to multiple
faults and finally crashs.

Depending on the architecture (AMD or Intel) pretended by
guests, various checks according to vendor's documentation
are implemented to overcome the current issue and behave
like the CPUs physical counterparts.

(Therefore using Intel's "Intel 64 and IA-32 Architecture Software
Developers Manual" http://www.intel.com/content/dam/doc/manual/
64-ia-32-architectures-software-developer-manual-325462.pdf
and AMD's "AMD64 Architecture Programmer's Manual Volume 3:
General-Purpose and System Instructions"
http://support.amd.com/us/Processor_TechDocs/APM_V3_24594.pdf )

Screenshots of an i686 testing VM (CORE i5 host) before
and after applying this patch are available under:

http://matrixstorm.com/software/linux/kvm/20111229/before.jpg
http://matrixstorm.com/software/linux/kvm/20111229/after.jpg

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
---
arch/x86/include/asm/kvm_emulate.h | 15 +++++++++
arch/x86/kvm/emulate.c | 57 ++++++++++++++++++++++++++++++++++++
2 files changed, 72 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index a026507..2a86ff9 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -297,6 +297,21 @@ struct x86_emulate_ctxt {
#define X86EMUL_MODE_PROT (X86EMUL_MODE_PROT16|X86EMUL_MODE_PROT32| \
X86EMUL_MODE_PROT64)

+/* CPUID vendors */
+#define X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx 0x68747541
+#define X86EMUL_CPUID_VENDOR_AuthenticAMD_ecx 0x444d4163
+#define X86EMUL_CPUID_VENDOR_AuthenticAMD_edx 0x69746e65
+
+#define X86EMUL_CPUID_VENDOR_AMDisbetter_ebx 0x69444d41
+#define X86EMUL_CPUID_VENDOR_AMDisbetter_ecx 0x21726574
+#define X86EMUL_CPUID_VENDOR_AMDisbetter_edx 0x74656273
+
+#define X86EMUL_CPUID_VENDOR_GenuineIntel_ebx 0x756e6547
+#define X86EMUL_CPUID_VENDOR_GenuineIntel_ecx 0x6c65746e
+#define X86EMUL_CPUID_VENDOR_GenuineIntel_edx 0x49656e69
+
+
+
enum x86_intercept_stage {
X86_ICTP_NONE = 0, /* Allow zero-init to not match anything */
X86_ICPT_PRE_EXCEPT,
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f1e3be1..78edb66 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1891,6 +1891,63 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
return emulate_ud(ctxt);

ops->get_msr(ctxt, MSR_EFER, &efer);
+ // check - if guestOS is aware of syscall (0x0f05)
+ if ((efer & EFER_SCE) == 0) return emulate_ud(ctxt);
+ else {
+ // ok, at this point it becomes vendor-specific
+ // so first get us an cpuid
+ struct kvm_cpuid_entry2 *vendor;
+
+ // eax = 0x00000000, ebx = 0x00000000 ==> cpu-vendor
+ vendor = kvm_find_cpuid_entry(emul_to_vcpu(ctxt), 0, 0);
+
+ if (likely(vendor)) {
+ // AMD (AuthenticAMD)/(AMDisbetter!)
+ if ((vendor->ebx==X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx &&
+ vendor->ecx==X86EMUL_CPUID_VENDOR_AuthenticAMD_ecx &&
+ vendor->edx==X86EMUL_CPUID_VENDOR_AuthenticAMD_edx) ||
+ (vendor->ebx==X86EMUL_CPUID_VENDOR_AMDisbetter_ebx &&
+ vendor->ecx==X86EMUL_CPUID_VENDOR_AMDisbetter_ecx &&
+ vendor->edx==X86EMUL_CPUID_VENDOR_AMDisbetter_edx)) {
+
+ // if cpu is not in longmode...
+ // ...check edx bit11 of cpuid 0x80000001
+ if (ctxt->mode != X86EMUL_MODE_PROT64) {
+ vendor= kvm_find_cpuid_entry(emul_to_vcpu(ctxt), 0x80000001, 0);
+ if (likely(vendor)) {
+ if (unlikely(((vendor->edx >> 11) & 0x1) == 0)) return emulate_ud(ctxt);
+ } else return emulate_ud(ctxt); // assuming there is no bit 11
+ }
+ goto __em_syscall_vendor_processed;
+ } // end "AMD"
+
+ // Intel (GenuineIntel)
+ // remarks: Intel CPUs only support "syscall" in 64bit longmode
+ // Also an 64bit guest within an 32bit compat-app running
+ // will #UD !!
+ // While this behaviour can be fixed (by emulating) into
+ // an AMD response - CPUs of AMD can't behave like Intel
+ // because without an hardware-raised #UD there is no
+ // call into emulation-mode (see x86_emulate_instruction(...)) !
+ // TODO: make AMD-behaviour configurable
+ if (vendor->ebx==X86EMUL_CPUID_VENDOR_GenuineIntel_ebx &&
+ vendor->ecx==X86EMUL_CPUID_VENDOR_GenuineIntel_ecx &&
+ vendor->edx==X86EMUL_CPUID_VENDOR_GenuineIntel_edx) {
+ if (ctxt->mode != X86EMUL_MODE_PROT64) return emulate_ud(ctxt);
+ goto __em_syscall_vendor_processed;
+ } // end "Intel"
+ } //end "likely(vendor)"
+
+ // default:
+ // this code wasn't able to process vendor
+ // so apply Intels stricter rules...
+ pr_unimpl(emul_to_vcpu(ctxt), "unknown vendor of cpu - assuming intel\n");
+ if (ctxt->mode != X86EMUL_MODE_PROT64) return emulate_ud(ctxt);
+
+ __em_syscall_vendor_processed:
+ vendor=NULL;
+ }
+
setup_syscalls_segments(ctxt, &cs, &ss);

ops->get_msr(ctxt, MSR_STAR, &msr_data);
--
1.7.3.4
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