[Qemu-devel] qemu(-dm): aborting on wrong mmio size?

[Qemu-devel] qemu(-dm): aborting on wrong mmio size?

[Laszlo Ersek]

Hi,

in the qemu-xen-unstable tree 
(git://xenbits.xen.org/qemu-xen-unstable.git), the do_inp() function 
[i386-dm/helper2.c] makes the process exit if the operand size is wrong. 
Blame: 6040eea5 ("More files imported from xen-unstable 
17192:59b8768d0d0d").

In the qemu tree (git://git.qemu.org/qemu.git), the do_inp() function 
[xen-all.c] does the same (via hw_error() / abort()). Blame: 9ce94e7c 
("xen: Initialize event channels and io rings").

Is it justified to kill the emulator when this happens (eg. memory 
mapped IO with 64-bit operand)? What would happen on real hardware? If 
it's "undefined", wouldn't it be "better" (for some definition of 
"better") to return a constant?

Thank you,
Laszlo

Re: [Qemu-devel] [Xen-devel] qemu(-dm): aborting on wrong mmio size?

[Jan Beulich]

>>> On 31.01.12 at 10:34, Laszlo Ersek <lersek@redhat.com> wrote:
> in the qemu-xen-unstable tree 
> (git://xenbits.xen.org/qemu-xen-unstable.git), the do_inp() function 
> [i386-dm/helper2.c] makes the process exit if the operand size is wrong. 
> Blame: 6040eea5 ("More files imported from xen-unstable 
> 17192:59b8768d0d0d").
> 
> In the qemu tree (git://git.qemu.org/qemu.git), the do_inp() function 
> [xen-all.c] does the same (via hw_error() / abort()). Blame: 9ce94e7c 
> ("xen: Initialize event channels and io rings").
> 
> Is it justified to kill the emulator when this happens (eg. memory 
> mapped IO with 64-bit operand)?

Afaict, this is not about MMIO, but PIO.

> What would happen on real hardware? If 
> it's "undefined", wouldn't it be "better" (for some definition of 
> "better") to return a constant?

The AMD manual specifies that REX.W is ignored; the Intel manual
doesn't mention REX at all here. However, if a decoder incorrectly
decodes the guest instruction, that's a bug there. So imo qemu
validly treats this condition as fatal.

Jan

Re: [Qemu-devel] [Xen-devel] qemu(-dm): aborting on wrong mmio size?

[Laszlo Ersek]

On 01/31/12 11:36, Jan Beulich wrote:
>>>> On 31.01.12 at 10:34, Laszlo Ersek<lersek@redhat.com>  wrote:
>> in the qemu-xen-unstable tree
>> (git://xenbits.xen.org/qemu-xen-unstable.git), the do_inp() function
>> [i386-dm/helper2.c] makes the process exit if the operand size is wrong.
>> Blame: 6040eea5 ("More files imported from xen-unstable
>> 17192:59b8768d0d0d").
>>
>> In the qemu tree (git://git.qemu.org/qemu.git), the do_inp() function
>> [xen-all.c] does the same (via hw_error() / abort()). Blame: 9ce94e7c
>> ("xen: Initialize event channels and io rings").
>>
>> Is it justified to kill the emulator when this happens (eg. memory
>> mapped IO with 64-bit operand)?
>
> Afaict, this is not about MMIO, but PIO.

One possible way seems to be (see 
http://xenbits.xensource.com/hg/linux-2.6.18-xen.hg/rev/1141):

vmx_hpw_miss() [xen/arch/ia64/vmx/vmx_fault.c]
-> emulate_io_inst() [xen/arch/ia64/vmx/mmio.c]
   -> mmio_access()
     -> legacy_io_access()
       -> vmx_send_assist_req() [xen/arch/ia64/vmx/vmx_support.c]
         -> notify_via_xen_event_channel() [xen/common/event_channel.c]

and in qemu-xen-unstable,

cpu_handle_ioreq() [i386-dm/helper2.c], set up in main_loop()
-> __handle_ioreq()
   -> cpu_ioreq_pio()
     -> do_inp()

Thanks,
Laszlo

Re: [Qemu-devel] [Xen-devel] qemu(-dm): aborting on wrong mmio size?

[Laszlo Ersek]

On 01/31/12 11:36, Jan Beulich wrote:
>>>> On 31.01.12 at 10:34, Laszlo Ersek<lersek@redhat.com>  wrote:

>> Is it justified to kill the emulator when this happens (eg. memory
>> mapped IO with 64-bit operand)?

> The AMD manual specifies that REX.W is ignored; the Intel manual
> doesn't mention REX at all here. However, if a decoder incorrectly
> decodes the guest instruction, that's a bug there. So imo qemu
> validly treats this condition as fatal.

 From the Itanium(R) SDM rev 2.3,

10.7.2.1 I/O Port Addressing Restrictions

     For the 64MB physical I/O port block the following operations are
     undefined and may result in unpredictable processor operation;
     references larger than 4-bytes, [...]

It seems that not only a decoding failure can trigger this.

Laszlo

Re: [Qemu-devel] [Xen-devel] qemu(-dm): aborting on wrong mmio size?

[Jan Beulich]

>>> On 31.01.12 at 12:04, Laszlo Ersek <lersek@redhat.com> wrote:
> On 01/31/12 11:36, Jan Beulich wrote:
>>>>> On 31.01.12 at 10:34, Laszlo Ersek<lersek@redhat.com>  wrote:
> 
>>> Is it justified to kill the emulator when this happens (eg. memory
>>> mapped IO with 64-bit operand)?
> 
>> The AMD manual specifies that REX.W is ignored; the Intel manual
>> doesn't mention REX at all here. However, if a decoder incorrectly
>> decodes the guest instruction, that's a bug there. So imo qemu
>> validly treats this condition as fatal.
> 
>  From the Itanium(R) SDM rev 2.3,
> 
> 10.7.2.1 I/O Port Addressing Restrictions
> 
>      For the 64MB physical I/O port block the following operations are
>      undefined and may result in unpredictable processor operation;
>      references larger than 4-bytes, [...]
> 
> It seems that not only a decoding failure can trigger this.

Oh, yes, I forgot that port I/O goes via MMIO on ia64. So yes, for
that case your observation is correct, though killing qemu-dm in
that case still seems to fall well under "undefined behavior". But
improving the situation wouldn't be bad then.

Jan