Re: [Xen-devel] rdmsr_safe in Linux PV (under Xen) gets an #GP:Re: [Fedora-xen] Running fedora xen on top of KVM?

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Sep 22, 2015 at 11:23 AM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Sun, Sep 20, 2015 at 09:49:04PM -0700, Andy Lutomirski wrote:
>> On Fri, Sep 18, 2015 at 12:04 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>> > On Fri, Sep 18, 2015 at 08:20:46AM -0700, Andy Lutomirski wrote:
>> >> In any event, Borislav, you must have typed rdmsr_safe for a reason :)
>> >
>> > Wasn't me:
>> >
>> > 6c62aa4a3c12 ("x86: make amd.c have 64bit support code")
>> >
>> > I think the error handling of rdmsrl_safe() was needed to do the pfn
>> > games which are done in the if-clause.
>>
>> I just tried it.  rdmsrl_safe and friends definitely work fine in that
>> code.  I think that Linux's Xen startup code is buggy and fails to set
>> up early exception handling.
>>
>> Try this (horribly whitespace damaged):
>>
>>  static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>>  {
>> +       u64 tmp;
>>  #ifdef CONFIG_X86_64
>>         c->x86_clflush_size = 64;
>>         c->x86_phys_bits = 36;
>> @@ -752,6 +753,9 @@ static void __init early_identify_cpu(struct cpuinfo_x86 
>> *c)
>>         c->cpu_index = 0;
>>         filter_cpuid_features(c, false);
>>
>> +       pr_err("trying to crash\n");
>> +       rdmsrl_safe(0x12345678, &tmp);
>> +
>>
>> It works fine.  I bet it crashes on a Xen guest, though.  I assume
>> that Xen just works in most cases by luck.
>
> (d31) mapping kernel into physical memory
> (d31) about to get started...
> (XEN) traps.c:3151: GPF (0000): ffff82d0801a31ed -> ffff82d08023c77b
> (XEN) traps.c:459:d31v0 Unhandled general protection fault fault/trap [#13] 
> on VCPU 0 [ec=0000]
> (XEN) domain_crash_sync called from entry.S: fault at ffff82d080238213 
> create_bounce_frame+0x12b/0x13a
> (XEN) Domain 31 (vcpu#0) crashed on cpu#35:
> (XEN) ----[ Xen-4.5.0  x86_64  debug=n  Not tainted ]----
> (XEN) CPU:    35
> (XEN) RIP:    e033:[<ffffffff81041b64>]
> (XEN) RFLAGS: 0000000000000246   EM: 1   CONTEXT: pv guest
> (XEN) rax: 0000000000000000   rbx: ffffffff81c03e64   rcx: 0000000012345678
> (XEN) rdx: ffffffff81c03de8   rsi: ffffffff81c03dec   rdi: 0000000012345278
> (XEN) rbp: ffffffff81c03e48   rsp: ffffffff81c03dd0   r8:  7420676e69797274
> (XEN) r9:  6873617263206f74   r10: 0000000000000000   r11: 0000000000000000
> (XEN) r12: 0000000012345678   r13: ffffffff81c03f00   r14: 0000000000000000
> (XEN) r15: 0000000000000000   cr0: 000000008005003b   cr4: 00000000001526f0
> (XEN) cr3: 00000014e8c97000   cr2: 0000000000000000
> (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
> (XEN) Guest stack trace from rsp=ffffffff81c03dd0:
> (XEN)    0000000012345678 0000000000000000 0000000000000000 ffffffff81041b64
> (XEN)    000000010000e030 0000000000010046 ffffffff81c03e18 000000000000e02b
> (XEN)    ffffffff81041b5d ffffffff81c03e48 0000000000811809 0000000000000000
> (XEN)    00000000000001a0 0000000001000000 ffffffff82009000 ffffffff81c03e68
> (XEN)    ffffffff81d211ea 0000000000000000 0000000000000000 ffffffff81c03ed8
> (XEN)    ffffffff81d1be59 ffffffff81c03ed8 ffffffff811892ab 0000000000000010
> (XEN)    ffffffff81c03ee8 ffffffff81c03ea8 697a696c61697469 ffffffff81f15442
> (XEN)    ffffffffffffffff ffffffff81db3900 0000000000000000 0000000000000000
> (XEN)    0000000000000000 ffffffff81c03f28 ffffffff81d10f0a 0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000 ffffffff81c03f38
> (XEN)    ffffffff81d10603 ffffffff81c03ff8 ffffffff81d15f5c 0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
> (XEN)    0000000000000000 ffd83a031f898b75 0000000022400800 0000000000000001
> (XEN)    0000000000000000 0000000000000000 00010102464c457f 0000000000000000
> (XEN)    00000001003e0003 0000000000000940 0000000000000040 00000000000012a0
> (XEN)    0038004000000000 0011001200400004 0000000500000001 0000000000000000
> [root@ovs107 ~]#
>
> (gdb) x/20i 0xffffffff81041b64
>    0xffffffff81041b64:  rdmsr
>

Exactly.  I think that Xen is missing code to wire up early_fixup_exception.

--Andy

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel