Date: Fri, 15 May 2009 08:04:36 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: alc@freebsd.org, Dag-Erling =?utf-8?q?Sm=C3=B8rgrav?= <des@des.no>, Alan Cox <alc@cs.rice.edu>, arch@freebsd.org Subject: Re: PTE modified bit emulation trap Message-ID: <200905150804.36977.jhb@freebsd.org> In-Reply-To: <4A0C49FF.1070707@cs.rice.edu> References: <86tz3o4lb9.fsf@ds4.des.no> <86prec4kwj.fsf@ds4.des.no> <4A0C49FF.1070707@cs.rice.edu>
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On Thursday 14 May 2009 12:42:39 pm Alan Cox wrote:
> Dag-Erling Sm=C3=B8rgrav wrote:
> > [from -alpha, -hackers]
> >
> > Dag-Erling Sm=C3=B8rgrav <des@des.no> writes:
> > =20
> >> Coverity complains about the lack of error checking in the following
> >> code in sys/kern/kern_sysctl.c, around line 1390:
> >>
> >> /*
> >> * Touch all the wired pages to avoid PTE modified
> >> * bit emulation traps on Alpha while holding locks
> >> * in the sysctl handler.
> >> */
> >> for (i =3D (wiredlen + PAGE_SIZE - 1) / PAGE_SIZE,
> >> cp =3D req->oldptr; i > 0; i--, cp +=3D PAGE_SIZE) {
> >> copyin(cp, &dummy, 1);
> >> copyout(&dummy, cp, 1);
> >> }
> >>
> >> Since Alpha is dead, can we remove this, or is it still needed for oth=
er
> >> platforms?
> >> =20
> >
> > kmacy suggested you might be the right person to ask... the conclusion
> > so far is that it *might* be necessary on sparc64 and / or mips.
> > =20
>=20
> I think that this code may no longer be needed, but I want to=20
> double-check. I faced a related problem implementing superpages=20
> support, so I introduced an additional "access type" parameter to=20
> pmap_enter(). This parameter was specifically intended to allow a=20
> pmap_enter() implementation to preset the PTE's modified bit. I think=20
> that the simulated page fault that occurs on vslock()-style wiring=20
> passes "write access" to pmap_enter(). If so, then it's just a matter=20
> of tweaking the MIPS or any other pmap_enter() to actually do something=20
> with the "access type" parameter. Currently, only the architectures=20
> that implement the pmap-level support for superpages, i.e., amd64 and=20
> i386, do anything with this parameter.
Then it sounds like the code should definitely be removed and that if any=20
problems do crop up, they can be fixed in pmap_enter() instead.
=2D-=20
John Baldwin
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