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Re: [RFC PATCH v2] xen/arm: improve handling of load/store instruction decoding
On Thu, 07 Mar 2024 12:43, Michal Orzel <michal.orzel@xxxxxxx> wrote:
On 07/03/2024 11:02, Manos Pitsidianakis wrote:
Hi Michal, Alex,
I'm responding to Michel but also giving my own review comments here.
On Thu, 07 Mar 2024 10:40, Michal Orzel <michal.orzel@xxxxxxx> wrote:
Hi Alex,
NIT: RFC tag is no longer needed.
On 06/03/2024 17:56, Alex Bennée wrote:
While debugging VirtIO on Arm we ran into a warning due to memory
being memcpy'd across MMIO space. While the bug was in the mappings
the warning was a little confusing:
(XEN) d47v2 Rn should not be equal to Rt except for r31
(XEN) d47v2 unhandled Arm instruction 0x3d800000
(XEN) d47v2 Unable to decode instruction
The Rn == Rt warning is only applicable to single register load/stores
so add some verification steps before to weed out unexpected accesses.
While at it update the Arm ARM references to the latest version of the
documentation.
Signed-off-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
Cc: Manos Pitsidianakis <manos.pitsidianakis@xxxxxxxxxx>
Move the CC line after --- so that it's not included in the final commit msg.
---
v2
- use single line comments where applicable
- update Arm ARM references
- use #defines for magic numbers
---
xen/arch/arm/decode.c | 35 ++++++++++++++++++++------
xen/arch/arm/decode.h | 57 ++++++++++++++++++++++++++++++++++++++-----
2 files changed, 79 insertions(+), 13 deletions(-)
diff --git a/xen/arch/arm/decode.c b/xen/arch/arm/decode.c
index 2537dbebc1..73a88e4701 100644
--- a/xen/arch/arm/decode.c
+++ b/xen/arch/arm/decode.c
@@ -87,15 +87,36 @@ static int decode_arm64(register_t pc, mmio_info_t *info)
return 1;
}
+ /* Check this is a load/store of some sort */
+ if ( (opcode.top_level.op1 & TL_LDST_OP1_MASK) != TL_LDST_OP1_VALUE )
+ {
+ gprintk(XENLOG_ERR, "Not a load/store instruction op1=%u\n",
+ opcode.top_level.op1);
+ goto bad_loadstore;
+ }
+
+ /* We are only expecting single register load/stores */
+ if ( (opcode.ld_st.op0 & LS_SREG_OP0_MASK) != LS_SREG_OP0_VALUE )
+ {
+ gprintk(XENLOG_ERR, "Not single register load/store op0=%u\n",
NIT: missing 'a' between Not and single
+ opcode.ld_st.op0);
+ goto bad_loadstore;
+ }
+
/*
- * Refer Arm v8 ARM DDI 0487G.b, Page - C6-1107
- * "Shared decode for all encodings" (under ldr immediate)
- * If n == t && n != 31, then the return value is implementation defined
- * (can be WBSUPPRESS, UNKNOWN, UNDEFINED or NOP). Thus, we do not support
- * this. This holds true for ldrb/ldrh immediate as well.
+ * Refer Arm v8 ARM DDI 0487J.a, Page - K1-12586
+ *
+ * STR (immediate) CONSTRAINED UNPREDICTABLE behaviour
+ *
+ * "If the instruction encoding specifies pre-indexed addressing or
+ * post-indexed addressing, and n == t && n != 31, then one of the
+ * following behaviors must occur:" UNDEFINED, NOP or UNKNOWN
+ *
+ * Execution @ EL0/EL1 when HCR_EL2.TIDCP is 1 traps to EL2 with
+ * EC = 0.
*
- * Also refer, Page - C6-1384, the above described behaviour is same for
- * str immediate. This holds true for strb/strh immediate as well
+ * This also hold true for LDR (immediate), Page K1-12581 and
+ * the RB/RH variants of both.
*/
if ( (opcode.ldr_str.rn == opcode.ldr_str.rt) && (opcode.ldr_str.rn != 31)
)
{
diff --git a/xen/arch/arm/decode.h b/xen/arch/arm/decode.h
index 13db8ac968..188114a71e 100644
--- a/xen/arch/arm/decode.h
+++ b/xen/arch/arm/decode.h
@@ -24,17 +24,54 @@
#include <asm/processor.h>
/*
- * Refer to the ARMv8 ARM (DDI 0487G.b), Section C4.1.4 Loads and Stores
- * Page 318 specifies the following bit pattern for
- * "load/store register (immediate post-indexed)".
+ * Refer to the ARMv8 ARM (DDI 0487J.a)
*
- * 31 30 29 27 26 25 23 21 20 11 9 4 0
+ * Section C A64 Instruct Set Encoding
This line is not needed
I think it is needed for context (it answers the question "what is
C4.1?" in the following line.
+ *
+ * C4.1 A64 instruction set encoding:
NIT: I would put a comma after section number i.e. C4.1, A64 ...
The same would apply in other places.
Style manuals use either space (like here), a period (.) or colon (:),
never a comma.
Since it's a NIT, I'm not going to object. I just care about readability, we do
not
need to adhere to any "style manuals".
I agree about readability :) the manuals mention was not an appeal to
authority, just a sign of what is more common out there hence readable
for more people. It is a nitpicking and subjective of course, so I'm not
arguing for/against it, just sharing my 2 cents.
+ *
+ * 31 30 29 28 25 24 0
* ___________________________________________________________________
- * |size|1 1 1 |V |0 0 |opc |0 | imm9 |0 1 | Rn | Rt |
- * |____|______|__|____|____|__|_______________|____|_________|_______|
+ * |op0 | x x | op1 | |
+ * |____|______|______|_______________________________________________|
+ *
+ * op0 = 0 is reserved
I'm not sure how to read it. It is reserved provided op1 is also 0.
Yes, it should say something like:
Decode field values op0 = 0, op1 = 0 are reserved.
Values op0 = 1, op1 = x1x0 correspond to Loads and Stores
+ * op1 = x1x0 for Loads and Stores
+ *
+ * Section C4.1.88 Loads and Stores
Missing colon at the end?
It's a heading so a colon would not be appropriate.
In this case why was it added before in:
"C4.1 A64 instruction set encoding:"
It should be removed from that, good point. Or at least put a colon here
and in all headers for consistency.
+ *
+ * 31 28 27 26 25 24 23 22 21 16 15 12 11 10 9 0
+ * ___________________________________________________________________
+ * | op0 | 1 | op1 | 0 | op2 | | op3 | | op4 | |
+ * |________|___|_____|___|_____|__|__________|______|_____|__________|
+ *
Maybe we should add the op{0,1,2,3,4} values for consistency?
Values op0=xx11, op1=0, op2=0x, op3=0xxxxx, op4=01 correspond to
Load/store register (immediate post-indexed)
I think this should stay neutral in case we add a new emulation in a future.
Do you mean for future Arm versions? decode.{c,h} should definitely be
more future-proof... I think it's okay in this case only because the
comment block starts with the source's name "ARMv8 ARM (DDI 0487J.a)". I
don't object however to what you're saying, either is fine for me!
+ * Page C4-653 Load/store register (immediate post-indexed)
+ *
+ * 31 30 29 27 26 25 24 23 22 21 20 12 11 10 9 5 4 0
+ * ___________________________________________________________________
+ * |size|1 1 1 |V | 0 0 | opc |0 | imm9 | 0 1 | Rn | Rt |
+ * |____|______|__|_____|_____|__|_______________|_____|______|_______|
*/
union instr {
uint32_t value;
+ struct {
+ unsigned int ign2:25;
Here, your numeration of ignore fields is in descending order (starting from
lsb) but ..,
+ unsigned int op1:4; /* instruction class */
+ unsigned int ign1:2;
+ unsigned int op0:1; /* value = 1b */
Why op0 = 0b1 ? This structure represents the generic bit layout (the emulation
deals with single ldr/str).
I would drop this comment.
It is a reserved bit which can never be 0.
Where did you take this information from?
As I wrote above, I don't think we should bind this union to a single
use case we currently have.
The struct top_level should represent the generic encoding of A64 instruction.
C4.1, page C4-400. op0 is only zero in the reserved (unallocated) case,
for the generic encoding.
~Michal
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