Messages in this thread | | | Date | Sun, 6 Sep 2020 02:22:28 +0200 | From | Luc Van Oostenryck <> | Subject | Re: [OpenRISC] [PATCH v2 3/3] openrisc: Fix issue with get_user for 64-bit values |
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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.
-- Luc
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