Re: [uml-devel] SKAS mode not working, x86_64 SMP host, x86_64 guest , problems with noprocmm

[Nikola Ciprich <nikola.ciprich@linuxbox.cz>]

> You really changed long to int? In this case, you may only have introduced a 
> bigger overflow.

Yes, I'm aware of that, I just wanted to try how i386 variant behaves.
btw I just tried gdb to find out why i386 UML on x86_64 host hangs just
at the beginning, and it turned out to be same problem with
__const_udelay. well, it's not a big surprise now I guess.
So if You need further testing, please feel free to post patches etc ;)
regards
nik.


On Wed, 2006-10-18 at 01:39 +0200, Blaisorblade wrote:
> On Tuesday 17 October 2006 22:21, Nikola Ciprich wrote:

> You really changed long to int? In this case, you may only have introduced a 
> bigger overflow.
> Jeff, I remember vaguely a recent (>=2.6.17) patch about udelay fixing a 
> misbehaviour, would you please complete my recollection and verify the 
> interaction with this?
> 
> Also, since we take __const_udelay and __udelay() from asm/arch/delay.h:
> (for i386):
> #define udelay(n) (__builtin_constant_p(n) ? \
>         ((n) > 20000 ? __bad_udelay() : __const_udelay((n) * 0x10c6ul)) : \
>         __udelay(n))
> 
> #define ndelay(n) (__builtin_constant_p(n) ? \
>        ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
>        __ndelay(n))
> 
> We then do _not_ take these definitions:
> arch/i386/lib/delay.c:
> 
> void __udelay(unsigned long usecs)
> {
>         __const_udelay(usecs * 0x000010c7); /* 2**32 / 1000000 (rounded up) */
> }
> 
> void __ndelay(unsigned long nsecs)
> {
>         __const_udelay(nsecs * 0x00005); /* 2**32 / 1000000000 (rounded up) */
> }
> 
> instead for us __udelay and __const_udelay match, which is not likely to help 
> at all. Their code is correct because the asm "mull" call divides the 
> obtained product by 2^32 by throwing away the 32 low bits (i.e. EAX, since 
> the mull result is in EDX:EAX), and the meaning of this is to do the division 
> only at the very end (to improve accuracy).
> 
> This bug is here at least since 2.6.16.
> Suggested solution:
> 1) have _our own_ delay.h, to avoid depending on subarchs' changes
> 2) first avoid playing so complex tricks, then maybe copy their implementation 
> (note it is slightly different already among x86 and x86_64).
> 
> > But I'm sure this is not the right solution, so what 
> > do You guys suggest? And also is really i386 variant safely implemented?
> 
> > thanks a lot!
> > nik.
> 


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