Re: [Qemu-devel] cpu_io_recompile, icount, and re-issued instructions

[Michael Eager]

On 05/02/2017 12:59 AM, Paolo Bonzini wrote:
> On 01/05/2017 03:57, Michael Eager wrote:
> > I'm seeing incorrect values when there is a write to a memory-mapped I/O
> > device when icount is set.  What I see happening is that a TB with ~20
> > instructions is executed which contains a write to the MM I/O address.
> > When it gets to the io_write routine, can_do_io is false, which results
> > in a call to cpu_io_recompile.
> >
> > cpu_io_recompile does what it (sort of) says it is supposed to do: it
> > builds a new TB with the I/O instruction as the last instruction in the
> > block, then re-issues the TB.  The problem is that the new TB contains
> > the instructions before the I/O instruction, so they are executed a
> > second time.
>
> They shouldn't.  When called from cpu_io_recompile,
> cpu_restore_state_from_tb should compute the I/O instruction's target PC
> from the host PC (stored in retaddr).
>
> Then what happens is the following:
>
> - cpu_io_recompile generates a new TB ending with the I/O instruction.
> This new TB has a hash table conflict with the old TB (same
> PC/cs_base/flags) the old TB is implicitly removed
>
> - cpu_io_recompile calls cpu_loop_exit_noexc, which goes back to the
> execution loop with updated PC
>
> - because the PC is different, a new TB is looked up for the I/O
> instruction's PC.  The TB probably is not there and translation starts
> again, this time at the I/O instruction
>
> - the new TB, when executed, causes cpu_io_recompile to fire again.
> This is the inefficient part mentioned in cpu_io_recompile
>
> - cpu_io_recompile now compiles a one-instruction TB and goes back to
> the execution loop
>
> - finally the execution loop executes the one-instruction TB for the I/O
> instruction, then it can go on

Thanks for the explanation.

The old QEMU source base I'm working with predates the refactoring of 
cpu_restore_state
by a couple months (sigh).  It also doesn't have the patches which changed the 
argument
from env to cpu.  I tried to cherry-pick the patches but there are too many 
conflicts.

I patched the old code to do what I suggested: a new TB for the already executed
instructions and a TB for the I/O instruction.  It works and has the small 
advantage
of not requiring a second trip through cpu_io_recompile.  But it will be a merge
conflict when we (eventually) upgrade to more recent QEMU sources.


--
Michael Eager    address@hidden
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