Re: Possibility of conflicting memory types in lazier TLB mode?

From: Andy Lutomirski
Date: Tue May 26 2020 - 20:10:15 EST

[cc Andrew Cooper and Dave Hansen]

On Fri, May 15, 2020 at 7:35 PM Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
> Excerpts from Rik van Riel's message of May 16, 2020 5:24 am:
> > On Fri, 2020-05-15 at 16:50 +1000, Nicholas Piggin wrote:
> >>
> >> But what about if there are (real, not speculative) stores in the
> >> store
> >> queue still on the lazy thread from when it was switched, that have
> >> not
> >> yet become coherent? The page is freed by another CPU and reallocated
> >> for something that maps it as nocache. Do you have a coherency
> >> problem
> >> there?
> >>
> >> Ensuring the store queue is drained when switching to lazy seems like
> >> it
> >> would fix it, maybe context switch code does that already or you
> >> have
> >> some other trick or reason it's not a problem. Am I way off base
> >> here?
> >
> > On x86, all stores become visible in-order globally.
> >
> > I suspect that
> > means any pending stores in the queue
> > would become visible to the rest of the system before
> > the store to the "current" cpu-local variable, as
> > well as other writes from the context switch code
> > become visible to the rest of the system.
> >
> > Is that too naive a way of preventing the scenario you
> > describe?
> >
> > What am I overlooking?
> I'm concerned if the physical address gets mapped with different
> cacheability attributes where that ordering is not enforced by cache
> coherency
> "The PAT allows any memory type to be specified in the page tables, and
> therefore it is possible to have a single physical page mapped to two
> or more different linear addresses, each with different memory types.
> Intel does not support this practice because it may lead to undefined
> operations that can result in a system failure. In particular, a WC
> page must never be aliased to a cacheable page because WC writes may
> not check the processor caches." -- Vol. 3A 11-35
> Maybe I'm over thinking it, and this would never happen anyway because
> if anyone were to map a RAM page WC, they might always have to ensure
> all processor caches are flushed first anyway so perhaps this is just a
> non-issue?

After talking to Andrew Cooper (hi!), I think that, on reasonably
modern Intel machines, WC memory is still *coherent* with the whole
system -- it's just not ordered the usual way. So I'm not convinced
there's an actual problem here. I don't know about AMD.