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Re: [Xen-devel] [PATCH v2 2/4] arm: processor: add new struct hsr_smc32 into hsr union

On 11/08/17 14:26, Volodymyr Babchuk wrote:

On 10.08.17 00:22, Julien Grall wrote:

On 09/08/2017 22:06, Volodymyr Babchuk wrote:
Hi Julien,

On 09.08.17 23:34, Julien Grall wrote:

On 09/08/2017 20:44, Volodymyr Babchuk wrote:
On ARMv8, one of conditional exceptions (SMC that originates
from aarch32 state) have extra field in HCR.ISS encoding:


And the register is called HSR and not HCR.

Indicates whether the instruction might have failed its condition
code check.
   0 - The instruction was unconditional, or was conditional and
   passed  its condition code check.
   1 - The instruction was conditional, and might have failed its
   condition code check.
(ARM DDI 0487A.k page D7-1949)

Please use the latest ARM ARM.

This is instruction specific field, so better to add new structure

This is an instruction...

to union hsr. This structure describes ISS encoding for an exception
from SMC instruction execution in AArch32 state. But we define this
struct for both ARMv7 and ARMv8. The reason is described in comment
to the structure:

"Nevertheless, we define this encoding for both ARMv7 and ARMv8,
because check_conditional_inst() should properly handle SMC
instruction in all modes: ARMv7, aarch32 and aarch64."

Hmmm. There are only two existing modes: AArch32 and AArch64. ARMv7 is
just a version of the specification which happen to only support
Yeah, I wondered how to formulate that better. Problem is that ARMv7
specification does not use term "AArch32". So I decided to mention ARMv7

The term AArch32 was introduced with ARMv8 and use to refer 32-bit
state. ARMv7 is only 32-bit, and therefore has only AArch32 state.
Hmm, maybe it is only me, but when I see term "AArch32" I automatically
think about ARMv8 only, because I know that there was no such term in
ARMv7. So for me "AArch32 or AArch64 state" sounds like "It is
ARMv8-only thing, no ARMv7 there". Thus, I'd prefer to leave mention
about ARMv7 for clarity. Maybe, just phrase it differently.

You are right here. For me, they are the same because ARMv8 32-bit has been designed to be compatible with the former. I would tend to use ARM 32-bit and AArch32 interchangeably.

How about this: "check_conditional_inst() should properly handle SMC
instruction on both architectures (ARMv7 and ARMv8) while running in
aarch32 or aarch64 mode" ?

"ARMv8 allows to trap conditional SMC from AArch32 state even if the
condition check failed. Modify check_conditional_inst() to handle them."

Actually Xen does not care about ARMv8 vs ARMv7. It only care about
AArch32 vs AArch64.
Yes. And probably it can be problem in the future. Because, as we can
see, there are differences between ARMv7 and ARMv8.

I don't see any problem. Bits not used are usually made RES{0,1} to
allow later revision using them for new features.
Okay, I'll refrain from use word "problem" :)

There are a few differences between the both. But I would not call that a problem as they are mostly the same.

There are also difference between ARMv8.0, ARMv8.1, ARMv8.2. But they
always ensure backward compatibility on reading or a way to detect the
new feature if the kernel has to set/clear bits.

In the case of the ISS for SMC, the bits used are RES0, with the new
meaning 0 means the SMC is unconditional or the condition passed. This
is compatible with ARMv7 because conditional SMC are only trapped when
the condition check passed.
I'll rewrite comments in terms of backward compatibility then.


Julien Grall

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