linux-kernel - Re: Flushing user entries for kernel mappings in x86

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Message-Id: <7EDEB913-E4F8-481F-93D3-EE158641A366@gmail.com>
Date:   Wed, 29 Aug 2018 01:37:55 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Flushing user entries for kernel mappings in x86

at 8:45 PM, Andy Lutomirski <luto@...nel.org> wrote:

> On Tue, Aug 28, 2018 at 6:46 PM, Nadav Amit <nadav.amit@...il.com> wrote:
>> Hello Andy,
>> 
>> Is there a reason for __flush_tlb_one_kernel() to flush the PTE not only in
>> the kernel address space, but also in the user one (as part of
>> __flush_tlb_one_user)? [ I obviously regard the case when PTI is on ].
> 
> In most cases, probably not, but it's fairly cheap, I think.  And it
> makes it so that we're okay if the TLB entry we're flushing is used by
> the entry code.

Thanks. I assumed that’s the case. I don’t know how cheap they are
(especially if INVPCID is not supported) but I guess they are not that
frequent.