On 01/12/2020 12:24, Oleksandr wrote:
On 01.12.20 14:13, Julien Grall wrote:
Hi Oleksandr,
Hi Julien.
--- a/xen/include/asm-arm/traps.h
+++ b/xen/include/asm-arm/traps.h
@@ -83,6 +83,30 @@ static inline bool VABORT_GEN_BY_GUEST(const
struct cpu_user_regs *regs)
(unsigned long)abort_guest_exit_end == regs->pc;
}
+/* Check whether the sign extension is required and perform
it */
+static inline register_t sign_extend(const struct hsr_dabt
dabt, register_t r)
+{
+ uint8_t size = (1 << dabt.size) * 8;
+
+ /*
+ * Sign extend if required.
+ * Note that we expect the read handler to have zeroed the
bits
+ * outside the requested access size.
+ */
+ if ( dabt.sign && (r & (1UL << (size - 1))) )
+ {
+ /*
+ * We are relying on register_t using the same as
+ * an unsigned long in order to keep the 32-bit assembly
+ * code smaller.
+ */
+ BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
+ r |= (~0UL) << size;
If `size` is 64, you will get undefined behavior there.
I think, we don't need to worry about undefined behavior here. Having
size=64 would be possible with doubleword (dabt.size=3). But if "r"
adjustment gets called (I mean Syndrome Sign Extend bit is set) then
we deal with byte, halfword or word operations (dabt.size<3). Or I
missed something?
At which point please put in a respective ASSERT(), possibly amended
by a brief comment.
ASSERT()s are only meant to catch programatic error. However, in
this case, the bigger risk is an hardware bug such as advertising a
sign extension for either 64-bit (or 32-bit) on Arm64 (resp. Arm32).
Actually the Armv8 spec is a bit more blurry when running in AArch32
state because they suggest that the sign extension can be set even
for 32-bit access. I think this is a spelling mistake, but it is
probably better to be cautious here.
Therefore, I would recommend to rework the code so it is only called
when len < sizeof(register_t).
I am not sure I understand the recommendation, could you please
clarify (also I don't see 'len' being used here).
Sorry I meant 'size'. I think something like:
if ( dabt.sign && (size < sizeof(register_t)) &&
(r & (1UL << (size - 1)) )
{
}
Another posibility would be:
if ( dabt.sign && (size < sizeof(register_t)) )
{
/* find whether the sign bit is set and propagate it */
}
I have a slight preference for the latter as the "if" is easier to read.
In any case, I think this change should be done in a separate patch (I
don't mint whether this is done after or before this one).
ok, I got it, thank you for the clarification. Of course, I will do that
in a separate patch, since the current one is to avoid code duplication
only. BTW, do you have comments on this patch itself?
The series is in my TODO list. I will have a look once in a bit :).
Cheers,
--
Julien Grall
