AMD TLB errata, (Was: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings)

[Peter Zijlstra]

On Tue, Oct 29, 2019 at 02:00:24PM +0300, Kirill A. Shutemov wrote:
> On Tue, Oct 29, 2019 at 09:56:02AM +0100, Peter Zijlstra wrote:
> > On Tue, Oct 29, 2019 at 09:43:18AM +0300, Kirill A. Shutemov wrote:
> > > But some CPUs don't like to have two TLB entries for the same memory with
> > > different sizes at the same time. See for instance AMD erratum 383.
> > > 
> > > Getting it right would require making the range not present, flush TLB and
> > > only then install huge page. That's what we do for userspace.
> > > 
> > > It will not fly for the direct mapping. There is no reasonable way to
> > > exclude other CPU from accessing the range while it's not present (call
> > > stop_machine()? :P). Moreover, the range may contain the code that doing
> > > the collapse or data required for it...
> > > 
> > > BTW, looks like current __split_large_page() in pageattr.c is susceptible
> > > to the errata. Maybe we can get away with the easy way...
> > 
> > As you write above, there is just no way we can have a (temporary) hole
> > in the direct map.
> > 
> > We are careful about that other errata, and make sure both translations
> > are identical wrt everything else.
> 
> It's not clear if it is enough to avoid the issue. "under a highly specific
> and detailed set of conditions" is not very specific set of conditions :P

Yeah, I know ... :/ Tom is there any chance you could shed a little more
light on that errata?