linux-kernel - Re: [OpenRISC] [PATCH v2 3/3] openrisc: Fix issue with get

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Date:   Mon, 7 Sep 2020 06:00:41 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc:     Jonas Bonn <jonas@...thpole.se>,
        LKML <linux-kernel@...r.kernel.org>,
        openrisc@...ts.librecores.org, Greentime Hu <green.hu@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [OpenRISC] [PATCH v2 3/3] openrisc: Fix issue with get_user for
 64-bit values

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