Re: [PATCH V3 19/23] xen/arm: io: Abstract sign-extension

[Oleksandr <olekstysh@xxxxxxxxx>]

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?
--
Regards,

Oleksandr Tyshchenko