From: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
To: Stafford Horne <shorne@gmail.com>
Cc: Jonas Bonn <jonas@southpole.se>,
LKML <linux-kernel@vger.kernel.org>,
openrisc@lists.librecores.org, Greentime Hu <green.hu@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [OpenRISC] [PATCH v2 3/3] openrisc: Fix issue with get_user for 64-bit values
Date: Sun, 6 Sep 2020 02:22:28 +0200 [thread overview]
Message-ID: <20200906002228.mrbs7pdyrf5ooi3c@ltop.local> (raw)
In-Reply-To: <20200905213408.GI3562056@lianli.shorne-pla.net>
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
next prev parent reply other threads:[~2020-09-06 0:22 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-05 13:19 [PATCH v2 0/3] OpenRISC fixes for 5.9 Stafford Horne
2020-09-05 13:19 ` [PATCH v2 1/3] openrisc: Reserve memblock for initrd Stafford Horne
2020-09-05 13:25 ` Stafford Horne
2020-09-06 6:15 ` Mike Rapoport
2020-09-05 13:19 ` [PATCH v2 2/3] openrisc: Fix cache API compile issue when not inlining Stafford Horne
2020-09-05 13:19 ` [PATCH v2 3/3] openrisc: Fix issue with get_user for 64-bit values Stafford Horne
2020-09-05 13:57 ` Luc Van Oostenryck
2020-09-05 21:34 ` Stafford Horne
2020-09-06 0:22 ` Luc Van Oostenryck [this message]
2020-09-06 21:00 ` [OpenRISC] " Stafford Horne
2020-09-05 13:24 ` [PATCH v2 0/3] OpenRISC fixes for 5.9 Stafford Horne
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