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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 03/30/2017 03:56 PM, Razvan Cojocaru wrote:
> On 03/30/2017 03:05 PM, Jan Beulich wrote:
>> What I do note though is that you don't copy back the value
>> __cmpxchg() returns, yet that's what is needed. *map may
>> have changed again already.
> 
> Changing the code to:
> 
> 1162     ret = __cmpxchg(map, old, new, bytes);
> 1163
> 1164     if ( ret != old )
> 1165     {
> 1166         memcpy(p_old, &ret, bytes);
> 1167         rc = X86EMUL_CMPXCHG_FAILED;
> 1168     }
> 
> where ret is an unsigned long still triggers BSODs when I add my patch
> to yours. I'll need to dig deeper.

Nevermind, I've found the culprit: hvm_emulate_one_vm_event()'s code
needs to be wrapped in a loop that checks for X86EMUL_RETRY again, since
hvmemul_cmpxchg() may return RETRY even for some mapping problems, in
which case we again end up with the guest trying to re-execute an
emulable CMPXCHG.

However, this gets me back to my original problem when I "solved" it in
the same manner (looping until emulation succeeds) back when
hvmemul_cmpxchg() failures were reported with RETRY: eventually the
guest BSODs with code 101. RETRY failures are still possible coming from
the hvmemul_vaddr_to_mfn() code in my patch.

I wonder if I should just return X86EMUL_CMPXCHG_FAILED for all those as
well and just never end up returning RETRY from hvmemul_cmpxchg().


Thanks,
Razvan

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