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SubjectRe: [patch 51/60] x86/mm: Allow flushing for future ASID switches
On Mon, Dec 04, 2017 at 02:22:54PM -0800, Andy Lutomirski wrote:

> > +static inline void invalidate_pcid_other(void)
> > +{
> > + /*
> > + * With global pages, all of the shared kenel page tables
> > + * are set as _PAGE_GLOBAL. We have no shared nonglobals
> > + * and nothing to do here.
> > + */
> > + if (!static_cpu_has_bug(X86_BUG_CPU_SECURE_MODE_KPTI))
> > + return;
>
> I think I'd be more comfortable if this check were in the caller, not
> here. Shouldn't a function called invalidate_pcid_other() do what the
> name says?

Yeah, you're probably right. The thing is course that we only ever need
that operation for kpti (as of now). But me renaming this stuff made
this problem :/

> > + this_cpu_write(cpu_tlbstate.invalidate_other, true);
>
> Why do we need this extra variable instead of just looping over all
> other ASIDs and invalidating them? It would be something like:
>
> for (i = 1; i < TLB_NR_DYN_ASIDS; i++) {
> if (i != this_cpu_read(cpu_tlbstate.loaded_mm_asid))
> this_cpu_write(cpu_tlbstate.ctxs[i].ctx_id, 0);
> }
>
> modulo epic whitespace damage and possible typos.

I think the point is that we can do many invalidate_other's before we
ever do a switch_mm(). The above would be more expensive.

Not sure it would matter in practise though.

> > static inline void __flush_tlb_one(unsigned long addr)
> > {
> > count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ONE);
> > __flush_tlb_single(addr);
> > + /*
> > + * Invalidate other address spaces inaccessible to single-page
> > + * invalidation:
> > + */
>
> Ugh. If I'm reading this right, __flush_tlb_single() means "flush one
> user address" and __flush_tlb_one() means "flush one kernel address".

That would make sense, woulnd't it? :-) But afaict the __flush_tlb_one()
user in tlb_uv.c is in fact for userspace and should be
__flush_tlb_single().

Andrew, Mike, can either of you shed light on what exactly you need
invalidated there?

> That's, um, not exactly obvious. Could this be at least commented
> better?

As is __flush_tlb_single() does user and __flush_tlb_one() does
user+kernel.

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