Re: [Xen-devel] [PATCH RFC] x86/emulate: implement hvmemul_cmpxchg() with an actual CMPXCHG

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 04/04/2017 12:07 PM, Jan Beulich wrote:
>>>> On 03.04.17 at 20:36, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> Another log, making sure that the MMIO emulation fail really happens on
>> the same processor as the preceding (identical bytes) fail:
>>
>> (XEN) [8] gfn: 0x276f
>> (XEN) [11] virtual address: 0x8276f09c, rc: 0
>> (XEN) [8] virtual address: 0x8276f09c, rc: 4
>> (XEN) Dump follows for VCPU 8
>> (XEN) Mem event (RETRY) emulation failed: d3v8 32bit @ 0008:826a3861 ->
>> f0 0f ba 30 00 72 07 8b cb e8 da 4b ff ff 8b 45
> 
> So what does your code do following this? Namely is there any exit
> back to guest context prior to the retried operation succeeding (e.g.
> are you allowing the instruction to be re-executed)? If there is, I'd
> suspect you don't clean up some internal state and ...

That code is called from hvm_do_resume() ...

>> (XEN) [11] gfn: 0x276f
>> (XEN) [11] virtual address: 0x8276f0d0, rc: 0
>> (XEN) [11] gfn: 0x276f
>> (XEN) [11] virtual address: 0x8276f09c, rc: 0
>> (XEN) [11] gfn: 0x276f
>> (XEN) [11] virtual address: 0x8276f0cc, rc: 0
>> (XEN) [11] gfn: 0x276f
>> (XEN) [11] virtual address: 0x8276f09c, rc: 0
>> (XEN) [8] gfn: 0xfed00
>> (XEN) !page
>> (XEN) hvmemul_vaddr_to_mfn() fail
>> (XEN) [8] virtual address: 0xffd080f0, rc: 1
>> (XEN) Dump follows for VCPU 8
>> (XEN) MMIO emulation failed: d3v8 32bit @ 0008:8263df3c -> f0 0f ba 30
>> 00 72 07 8b cb e8 da 4b ff ff 8b 45
> 
> ... this then happens in the context of an interrupt taken on
> the instruction to be re-executed (i.e. before it actually gets
> re-executed). "Normal" retry processing is not supposed to
> reach guest context again, i.e. the retry is being performed
> from (iirc) hvm_do_resume(). Hence it can re-use previously
> collected information on the instruction and its operand(s).
> Any exit to guest context, otoh, would require invalidation of
> such internally cached state.

... which does nothing if hvm_emulate_one_vm_event() fails. It doesn't
even check the return code. That's why I have added the do { } while (
rc == X86EMUL_RETRY ); loop around the code in
hvm_emulate_one_vm_event() and noticed that the BSODs appear much later
(and they're timeout BSODs, presumably because of a taking-too-long
RETRY loop), or the guest becomes unresponsive.

I'm not sure what cached state that would be, I'll take a closer look at
the code.

I'll retest as soon as possible (it's been a very busy time, sorry for
the late replies).


Thanks,
Razvan

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