Re: [Qemu-devel] [PATCH 2/8] cpu: Define new cpu_transaction_failed() hook

[Richard Henderson]

On 08/04/2017 10:20 AM, Peter Maydell wrote:
> Currently we have a rather half-baked setup for allowing CPUs to
> generate exceptions on accesses to invalid memory: the CPU has a
> cpu_unassigned_access() hook which the memory system calls in
> unassigned_mem_write() and unassigned_mem_read() if the current_cpu
> pointer is non-NULL.  This was originally designed before we
> implemented the MemTxResult type that allows memory operations to
> report a success or failure code, which is why the hook is called
> right at the bottom of the memory system.  The major problem with
> this is that it means that the hook can be called even when the
> access was not actually done by the CPU: for instance if the CPU
> writes to a DMA engine register which causes the DMA engine to begin
> a transaction which has been set up by the guest to operate on
> invalid memory then this will casue the CPU to take an exception
> incorrectly.  Another minor problem is that currently if a device
> returns a transaction error then this won't turn into a CPU exception
> at all.
> 
> The right way to do this is to have allow the CPU to respond
> to memory system transaction failures at the point where the
> CPU specific code calls into the memory system.
> 
> Define a new QOM CPU method and utility function
> cpu_transaction_failed() which is called in these cases.
> The functionality here overlaps with the existing
> cpu_unassigned_access() because individual target CPUs will
> need some work to convert them to the new system. When this
> transition is complete we can remove the old cpu_unassigned_access()
> code.
> 
> Signed-off-by: Peter Maydell <address@hidden>
> ---
>  include/qom/cpu.h | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)

Reviewed-by: Richard Henderson <address@hidden>


r~