Re: [Xen-devel] Xen fails to boot inside QEMU on x86, no VMX:

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 23 Jan 2018, Jan Beulich wrote:
> >>> On 23.01.18 at 01:41, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 23/01/2018 00:38, Stefano Stabellini wrote:
> >> On Tue, 23 Jan 2018, Andrew Cooper wrote:
> >>> On 22/01/2018 23:48, Stefano Stabellini wrote:
> >>>> Hi all,
> >>>>
> >>>> Running Xen inside QEMU x86 without KVM acceleartion and without VMX
> >>>> emulation leads to the failure appended below.
> >>>>
> >>>> This trivial workaround "fixes" the problem:
> >>>>
> >>>> diff --git a/xen/arch/x86/extable.c b/xen/arch/x86/extable.c
> >>>> index 72f30d9..a67d6c1 100644
> >>>> --- a/xen/arch/x86/extable.c
> >>>> +++ b/xen/arch/x86/extable.c
> >>>> @@ -168,7 +168,6 @@ static int __init stub_selftest(void)
> >>>>                         _ASM_EXTABLE(.Lret%=, .Lfix%=)
> >>>>                         : [exn] "+m" (res)
> >>>>                         : [stb] "r" (addr), "a" (tests[i].rax));
> >>>> -        ASSERT(res == tests[i].res.raw);
> >>>>      }
> >>>>  
> >>>>      return 0;
> >>>>
> >>>>
> >>>> Any suggestions?
> >>> Which i failed?  This will probably be an emulation bug in Qemu.
> >> i=2 is the culprit
> > 
> > Qemu doesn't emulate %rsp-based memory accesses properly.  It should
> > raise #SS[0], and is presumably raising #GP[0] instead.
> 
> Right, the value on %rax supports that suspicion. Dropping the
> ASSERT() is no option, of course. If we were able to reliably
> detect that we're running under qemu, we could cater for this
> special case, but I can't seem to be able to think of other options
> besides adding a command line option allowing to bypass the self
> test.

I am going to give a look at the QEMU side of things. However, even if I
fix the bug in QEMU, it won't solve the problem for all the QEMU
instances already out there, shipped by distros, etc.

So, I think that regardless of the QEMU fix, we also need to add a
workaround in Xen. We can detect QEMU from the cpuid string, which is
going to be TCGTCGTCGTCG.

What do you think of something like below?


diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 6cf3628..471e394 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -280,6 +280,21 @@ static void __init early_cpu_detect(void)
        initialize_cpu_data(0);
 }
 
+bool tcg_workaround = false;
+static inline void hypervisor_cpuid_base(struct cpuinfo_x86 *c)
+{
+       uint32_t base, eax, signature[3];
+       char *sig = "TCGTCGTCGTCG";
+
+       if ( c->phys_proc_id )
+               return;
+
+       base = 0x40000000;
+       cpuid(base, &eax, &signature[0], &signature[1], &signature[2]);
+       if ( !memcmp(sig, signature, 12) )
+               tcg_workaround = true;
+}
+
 static void generic_identify(struct cpuinfo_x86 *c)
 {
        u32 eax, ebx, ecx, edx, tmp;
@@ -343,6 +358,8 @@ static void generic_identify(struct cpuinfo_x86 *c)
                            
&c->x86_capability[cpufeat_word(X86_FEATURE_FSGSBASE)],
                            &c->x86_capability[cpufeat_word(X86_FEATURE_PKU)],
                            
&c->x86_capability[cpufeat_word(X86_FEATURE_AVX512_4VNNIW)]);
+
+       hypervisor_cpuid_base(c);
 }
 
 /*
diff --git a/xen/arch/x86/extable.c b/xen/arch/x86/extable.c
index 6fffe05..6c8ca3d 100644
--- a/xen/arch/x86/extable.c
+++ b/xen/arch/x86/extable.c
@@ -146,6 +146,9 @@ static int __init stub_selftest(void)
     unsigned long addr = this_cpu(stubs.addr) + STUB_BUF_SIZE / 2;
     unsigned int i;
 
+    if ( tcg_workaround )
+        return 0;
+
     printk("Running stub recovery selftests...\n");
 
     for ( i = 0; i < ARRAY_SIZE(tests); ++i )
diff --git a/xen/include/asm-x86/processor.h b/xen/include/asm-x86/processor.h
index 41a8d8c..dfc4537 100644
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -556,6 +556,7 @@ int early_microcode_update_cpu(bool start_update);
 int early_microcode_init(void);
 int microcode_init_intel(void);
 int microcode_init_amd(void);
+extern bool tcg_workaround;
 
 enum get_cpu_vendor {
     gcv_host,

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