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

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Message-ID: <5bc39561-b65e-82fd-3218-d91a4d22613a@linux.intel.com>
Date:   Wed, 1 Nov 2017 13:59:17 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "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 11/01/2017 01:31 PM, Andy Lutomirski wrote:
> 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.

The point of the whole thing is to (relatively) efficiently flush
*kernel* TLB entries in *other* address spaces.  I did it way down in
the TLB handling functions because not everybody goes through
flush_tlb_func_common() to flush kernel addresses.

I used the variable instead of just invalidating the contexts directly
because I hooked into the __flush_tlb_single() path and it's used in
loops like this:

	for (addr = start; addr < end; addr++)
		__flush_tlb_single()

I didn't want to add a loop that effectively does:

	for (addr = start; addr < end; addr++)
		__flush_tlb_single();
		for (i = 0; i < TLB_NR_DYN_ASIDS; i++)
			this_cpu_write(cpu_tlbstate.ctxs[i].ctx_id, 0);

Even with just 6 ASIDS it seemed a little silly.  It would get _very_
silly if we ever decided to grow TLB_NR_DYN_ASIDS.