On Sun, Sep 06, 2020 at 02:22:28AM +0200, Luc Van Oostenryck wrote:
> On Sun, Sep 06, 2020 at 06:34:08AM +0900, Stafford Horne wrote:
> > On Sat, Sep 05, 2020 at 03:57:14PM +0200, Luc Van Oostenryck wrote:
> > > On Sat, Sep 05, 2020 at 10:19:35PM +0900, Stafford Horne wrote:
> > >
> > > Hi,
> > >
> > > The change for 64-bit get_user() looks good to me.
> > > But I wonder, given that openrisc is big-endian, what will happen
> > > you have the opposite situation:
> > > u32 *ptr;
> > > u64 val;
> > > ...
> > > get_user(val, ptr);
> > >
> > > Won't you end with the value in the most significant part of
> > > the register pair?
> >
> > Hi Luc,
> >
> > The get_user function uses the size of the ptr to determine how to do the
> > load ,
> > so this case would not use the 64-bit pair register logic. I think it
> > should be
> > ok, the end result would be the same as c code:
> >
> > var = *ptr;
>
> Hi,
>
> Sorry to insist but both won't give the same result.
> The problem comes from the output part of the asm: "=r" (x).
>
> The following code:
> u32 getp(u32 *ptr)
> {
> u64 val;
> val = *ptr;
> return val;
> }
> will compile to something like:
> getp:
> l.jr r9
> l.lwz r11, 0(r3)
>
> The load is written to r11, which is what is returned. OK.
>
> But the get_user() code with a u32 pointer *and* a u64 destination
> is equivalent to something like:
> u32 getl(u32 *ptr)
> {
> u64 val;
>
> asm("l.lwz %0,0(%1)" : "=r"(val) : "r"(ptr));
> return val;
> }
> and this compiles to:
> getl:
> l.lwz r17,0(r3)
> l.jr r9
> l.or r11, r19, r19
>
> The load is written to r17 but what is returned is the content of r19.
> Not good.
>
> I think that, in the get_user() code:
> * if the pointer is a pointer to a 64-bit quantity, then variable
> used in as the output in the asm needs to be a 64-bit variable
> * if the pointer is a pointer to a 32-bit quantity, then variable
> used in as the output in the asm needs to be a 64-bit variable
> At least one way to guarantee this is to use a temporary variable
> that matches the size of the pointer.
Hello,
Thanks for taking the time to explain. I see your point, it makes sense I will
fix this up.
-Stafford
