Is pmap_kextract() allowed to fault?
jake at locore.ca
Mon Apr 21 12:20:04 PDT 2003
Apparently, On Mon, Apr 21, 2003 at 11:33:22AM -0700,
Marcel Moolenaar said words to the effect of;
> On Mon, Apr 21, 2003 at 09:24:49AM -0400, Jake Burkholder wrote:
> > Apparently, On Sun, Apr 20, 2003 at 10:53:32PM -0700,
> > Marcel Moolenaar said words to the effect of;
> > > Gang,
> > >
> > > On ia64 pmap_kextract() uses the tpa instruction which given a
> > > virtual address returns the physical address based on the
> > > translation registers and cache (ie TLB). This can fault when
> > > there's currently no mapping for the virtual address.
> > >
> > > Since all other architectures have a non-faulting implementation
> > > (AFAICT), I'm a bit worried that we might get into trouble on
> > > ia64. I couldn't find anything about pmap_kextract(), so maybe
> > > anybody can enlighten me:
> > >
> > > 1. Is pmap_kextract() allowed to fault?
> > It depends what kind of fault. Will tpa fail if it causes a tlb fault
> > and the page is not in the vhpt (or whatever the fault handler searches),
> > or will it end up calling vm_fault and actually trying to fault in the
> > page?
> It will end up calling vm_fault() if so required.
Yes, this is very bad. Especially for things like /dev/kmem, where you
want to validate an address passed from userland.
> > > 2. Is pmap_kextract() used often enough that using the cpu's TLB
> > > is a possible performance speedup even if there are costly faults
> > > that can sometimes happen?
> > I doubt it.
> Ok, thanks. I think I'll use a non-trapping implementation then.
> There's just too much circumstantiality...
> Marcel Moolenaar USPA: A-39004 marcel at xcllnt.net
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