[Bug 1906295] Re: Implementation of exclusive monitor in ARM

[Peter Maydell]

QEMU's load/store exclusive implementation is known to not be
architecturally compliant. Most notably, it works on virtual addresses
and the architecture specifies that exclusives are supposed to work on
physical addresses. We provide a "good enough for what real code (not
weird test cases) needs" implementation.

However, in this specific case, we're within the architectural
requirements. In the ARMv8A Arm ARM (DDI0487F.c) the pseudocode
AArch32.ExclusiveMonitorsPass() function has this comment:

// It is IMPLEMENTATION DEFINED whether the detection of memory aborts happens
// before or after the check on the local Exclusives monitor. As a result a 
failure
// of the local monitor can occur on some implementations even if the memory
// access would give a memory about.

In other words, it's up to the implementation whether it detects and
reports the invalid address first. QEMU's implementation chooses not to.


** Changed in: qemu
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1906295

Title:
  Implementation of exclusive monitor in ARM

Status in QEMU:
  Invalid

Bug description:
  Hi

  I refer to the implementation of exclusive monitor in ARM32. For
  instruction like STREX Rx,Ry,[Rz], we need to check whether the
  address [Rz] is in exclusive state. If not, we set the value Rx as 1
  without doing the store operation. However, I noticed that QEMU will
  not check whether the address that Rz points to is a legal address or
  not. If the value of Rz is 0x0 but it is not in exclusive state. QEMU
  will set Rx as 1 and continue to execute the following instructions.

  However, physical devices will check the value of Rz. If Rz is an illegal 
address (e.g., 0x0), a SIGSEGV signal will be raised even the address is not in 
exclusive state. I searched many documentation about ARM and it seems that 
manual of ARM specification does not specify the implementation of exclusive 
monitor in detail. I am not sure which one is the right behavior. 
  Should QEMU add this check? This might not be a mistake. However, should it 
be better if QEMU has the same behavior as a physical device? Feel free if you 
need a testcase. Many thanks

  Regards
  Muhui

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