linux-kernel - Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes

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Message-ID: <154B90AD-7EC3-4B86-8061-D737A948A77C@vmware.com>
Date:   Tue, 27 Aug 2019 23:55:01 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes

> On Aug 27, 2019, at 4:13 PM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> On Fri, Aug 23, 2019 at 11:13 PM Nadav Amit <namit@...are.com> wrote:
>> INVPCID is considerably slower than INVLPG of a single PTE. Using it to
>> flush the user page-tables when PTI is enabled therefore introduces
>> significant overhead.
>> 
>> Instead, unless page-tables are released, it is possible to defer the
>> flushing of the user page-tables until the time the code returns to
>> userspace. These page tables are not in use, so deferring them is not a
>> security hazard.
> 
> I agree and, in fact, I argued against ever using INVPCID in the
> original PTI code.
> 
> However, I don't see what freeing page tables has to do with this.  If
> the CPU can actually do speculative page walks based on the contents
> of non-current-PCID TLB entries, then we have major problems, since we
> don't actively flush the TLB for non-running mms at all.

That was not my concern.

> 
> I suppose that, if we free a page table, then we can't activate the
> PCID by writing to CR3 before flushing things.  But we can still defer
> the flush and just set the flush bit when we write to CR3.

This was my concern. I can change the behavior so the code would flush the
whole TLB instead. I just tried not to change the existing behavior too
much.