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From: Andy Lutomirski <luto@kernel.org>
Date: Mon, 5 Jun 2017 15:44:10 -0700
Message-ID: <CALCETrWUWPUKi2zLESbQiFjYK_90-Cts3fYJh4nSact+g7zb_A@mail.gmail.com>
Subject: Re: [RFC 01/11] x86/ldt: Simplify LDT switching logic
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>, Borislav Petkov <bpetkov@suse.de>, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Nadav Amit <nadav.amit@gmail.com>, Rik van Riel <riel@redhat.com>

On Mon, Jun 5, 2017 at 3:40 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Mon, Jun 5, 2017 at 3:36 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> We used to switch the LDT if the prev and next mms' LDTs didn't
>> match.
>
> I think the "LDT didn't match" was really just a simpler and more
> efficient way to say "they weren't both NULL".

Once we go fully lazy (later in this series), though, I'd start
worrying that the optimization would be wrong:

1  Load ldt 0x1234
2. Become lazy
3. LDT changes twice from a remote cpu and the second change reuses
the pointer 0x1234.
4. We go unlazy, prev == next, but LDTR is wrong.

This isn't a bug in current kernels because step 3 will force a leave_mm().

>
> I think you actually broke that optimization, and it now does *two*
> tests instead of just one.

I haven't looked at the generated code, but shouldn't it be just orq; jnz?

--Andy

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