linux-kernel - Re: [RFC PATCH 0/3] x86/mm/tlb: Defer TLB flushes with PTI

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Message-Id: <1E9E8042-5FF8-4C06-9BE1-E0A7440E35AA@gmail.com>
Date:   Fri, 23 Aug 2019 23:09:26 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        the arch/x86 maintainers <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 0/3] x86/mm/tlb: Defer TLB flushes with PTI

Sorry, I made a mistake and included the wrong patches. I will send
RFC v2 in few minutes.


> On Aug 23, 2019, at 3:46 PM, Nadav Amit <namit@...are.com> wrote:
> 
> INVPCID is considerably slower than INVLPG of a single PTE, but it is
> currently used to flush PTEs in the user page-table when PTI is used.
> 
> Instead, it is possible to defer TLB flushes until after the user
> page-tables are loaded. Preventing speculation over the TLB flushes
> should keep the whole thing safe. In some cases, deferring TLB flushes
> in such a way can result in more full TLB flushes, but arguably this
> behavior is oftentimes beneficial.
> 
> These patches are based and evaluated on top of the concurrent
> TLB-flushes v4 patch-set.
> 
> I will provide more results later, but it might be easier to look at the
> time an isolated TLB flush takes. These numbers are from skylake,
> showing the number of cycles that running madvise(DONTNEED) which
> results in local TLB flushes takes:
> 
> n_pages		concurrent	+deferred-pti		change
> -------		----------	-------------		------
> 1		2119		1986 			-6.7%
> 10		6791		5417 			 -20%
> 
> Please let me know if I missed something that affects security or
> performance.
> 
> [ Yes, I know there is another pending RFC for async TLB flushes, but I
>  think it might be easier to merge this one first ]
> 
> Nadav Amit (3):
>  x86/mm/tlb: Defer PTI flushes
>  x86/mm/tlb: Avoid deferring PTI flushes on shootdown
>  x86/mm/tlb: Use lockdep irq assertions
> 
> arch/x86/entry/calling.h        | 52 +++++++++++++++++++--
> arch/x86/include/asm/tlbflush.h | 31 ++++++++++--
> arch/x86/kernel/asm-offsets.c   |  3 ++
> arch/x86/mm/tlb.c               | 83 +++++++++++++++++++++++++++++++--
> 4 files changed, 158 insertions(+), 11 deletions(-)
> 
> -- 
> 2.17.1