Re: [Qemu-devel] [PATCH v1 8/9] target-arm: A64: Create Instruction Syndromes for Data Aborts

[Peter Maydell <peter.maydell@linaro.org>]

On 18 February 2016 at 09:56, Edgar E. Iglesias
<edgar.iglesias@gmail.com> wrote:
> On Tue, Feb 16, 2016 at 07:13:32PM +0000, Peter Maydell wrote:
>> I think this patch also would be simpler if the encoded info
>> put in with the TBs was just the syndrome register, rather
>> than some other encoding.
>
> My first try was to only pass the bits needed for the iss
> (i.e not the full data abort syndrome). We don't have all
> the info needed at translation time to create the full
> syndrome (e.g stage2 trap? stage2 trap while stage1 PTW, etc).
>
> But we could maybe create as much of the data abort syndrome
> as possible at translation time and then have the exception
> handling code add the missing bits. We can then pass the
> preliminary syndrome from translation time to exception time
> in the std syndrome format. I can have a look and see what I
> can do if that makes more sense.

Yep, that was basically what I had in mind.

Am I right in thinking that at translate time we capture:
 IL ISV SAS SSE SRT SF AR
and then at exception time we determine:
 EC FnV EA CM S1PTW WnR DFSC
?

In that case I think you could reasonably have the info stored
with the TBs be "template syndrome >> 14" (since bits [13:0] are
all info determined at exception-time). The only reason for doing
this is that the encoded data is stored as sleb128 deltas between
lines so keeping the values numerically smaller should make them
take up a bit less space. (I have no idea how significant the
space saving would be.)

You should then be able to just have restore_state_to_opc()
write straight to env->exception.syndrome rather than needing
a new field in the cpu state.

thanks
-- PMM