linux-kernel - Re: [PATCH 21/23] x86, pcid, kaiser: allow flushing for future ASID switches

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Date:   Wed, 1 Nov 2017 13:31:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 21/23] x86, pcid, kaiser: allow flushing for future ASID switches

On Wed, Nov 1, 2017 at 7:17 AM, Dave Hansen <dave.hansen@...ux.intel.com> wrote:
> On 11/01/2017 01:03 AM, Andy Lutomirski wrote:
>>> This ensures that any futuee context switches will do a full flush
>>> of the TLB so they pick up the changes.
>> I'm convuced.  What was wrong with the old code?  I guess I just don't
>> see what the problem is that is solved by this patch.
>
> Instead of flushing *now* with INVPCID, this lets us flush *later* with
> CR3.  It just hijacks the code that you already have that flushes CR3
> when loading a new ASID by making all ASIDs look new in the future.
>
> We have to load CR3 anyway, so we might as well just do this flush then.

Would it make more sense to put it in flush_tlb_func_common() instead?

Also, I don't understand what clear_non_loaded_ctxs() is trying to do.
It looks like it's invalidating all the other logical address spaces.
And I don't see why you want a all_other_ctxs_invalid variable.  Isn't
the goal to mark a single ASID as needing a *user* flush the next time
we switch to user mode using that ASID?  Your code seems like it's
going to flush a lot of *kernel* PCIDs.

Can you explain the overall logic?