From: Denys Vlasenko <dvlasenk@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>, Ingo Molnar <mingo@kernel.org>
Cc: Denys Vlasenko <vda.linux@googlemail.com>,
Brian Gerst <brgerst@gmail.com>, Borislav Petkov <bp@alien8.de>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address
Date: Tue, 31 Mar 2015 19:31:54 +0200 [thread overview]
Message-ID: <551ADA0A.7050701@redhat.com> (raw)
In-Reply-To: <CALCETrVHMEM7kRnhu7aVu8UFMSjiMLZBo=7vNfgK4fQx2oZMmg@mail.gmail.com>
On 03/31/2015 07:08 PM, Andy Lutomirski wrote:
> On Tue, Mar 31, 2015 at 9:43 AM, Ingo Molnar <mingo@kernel.org> wrote:
>>
>> * Denys Vlasenko <vda.linux@googlemail.com> wrote:
>>
>>>> I guess they could optimize it by adding a single "I am a modern
>>>> OS executing regular userspace" flag to the descriptor [or
>>>> expressing the same as a separate instruction], to avoid all that
>>>> legacy crap that won't trigger on like 99.999999% of systems ...
>>>
>>> Yes, that would be a useful addition. Interrupt servicing on x86
>>> takes a non-negligible hit because of IRET slowness.
>>
>> But ... to react to your other patch: detecting the common easy case
>> and doing a POPF+RET ourselves ought to be pretty good as well?
>>
>> But only if ptregs->rip != the magic RET itself, to avoid recursion.
>>
>> Even with all those extra checks it should still be much faster.
>>
>
> I have a smallish preference for doing sti;ret instead, because that
> keeps the funny special case entirely localized to the NMI code
> instead of putting it in the IRQ exit path. I suspect that the
> performance loss is at most a cycle or two (we're adding a branch, but
> sti itself is quite fast).
>
> That being said, I could easily be convinced otherwise.
Let me try to convince you. sti is 6 cycles.
The patch atop your code would be:
movq RIP-ARGOFFSET(%rsp), %rcx
+ cmp $magic_ret, %rcx
+ je real_iret
- btr $9, %rdi
movq %rdi, (%rsi)
movq %rcx, 8(%rsi)
movq %rsi, ORIG_RAX-ARGOFFSET(%rsp)
popq_cfi %r11
popq_cfi %r10
popq_cfi %r9
popq_cfi %r8
popq_cfi %rax
popq_cfi %rcx
popq_cfi %rdx
popq_cfi %rsi
popq_cfi %rdi
popq %rsp
- jc 1f
popfq_cfi
+magic_ret:
retq
-1:
- popfq_cfi
- sti
- retq
It's a clear (albeit small) win: the branch is almost never taken,
and we do not need sti.
next prev parent reply other threads:[~2015-03-31 17:32 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-26 12:42 [PATCH] x86/asm/entry/64: better check for canonical address Denys Vlasenko
2015-03-26 18:45 ` Andy Lutomirski
2015-03-27 8:57 ` Borislav Petkov
2015-03-30 14:27 ` Denys Vlasenko
2015-03-30 14:30 ` Andy Lutomirski
2015-03-30 14:45 ` Andy Lutomirski
2015-03-27 8:11 ` Ingo Molnar
2015-03-27 10:45 ` Denys Vlasenko
2015-03-27 11:17 ` Ingo Molnar
2015-03-27 11:28 ` Brian Gerst
2015-03-27 11:34 ` Ingo Molnar
2015-03-27 12:14 ` Denys Vlasenko
2015-03-27 12:16 ` Ingo Molnar
2015-03-27 12:31 ` Denys Vlasenko
2015-03-28 9:11 ` Ingo Molnar
2015-03-29 19:36 ` Denys Vlasenko
2015-03-29 21:12 ` Andy Lutomirski
2015-03-29 21:46 ` Denys Vlasenko
2015-03-31 16:43 ` Ingo Molnar
2015-03-31 17:08 ` Andy Lutomirski
2015-03-31 17:31 ` Denys Vlasenko [this message]
2015-03-27 11:27 ` Brian Gerst
2015-03-27 11:31 ` Ingo Molnar
2015-03-27 21:37 ` Andy Lutomirski
2015-04-02 17:37 ` Denys Vlasenko
2015-04-02 18:10 ` Ingo Molnar
2015-04-21 16:27 Denys Vlasenko
2015-04-21 18:08 ` Andy Lutomirski
2015-04-23 15:10 ` Borislav Petkov
2015-04-23 15:41 ` Andy Lutomirski
2015-04-23 15:49 ` Borislav Petkov
2015-04-23 15:52 ` Andy Lutomirski
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=551ADA0A.7050701@redhat.com \
--to=dvlasenk@redhat.com \
--cc=bp@alien8.de \
--cc=brgerst@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=mingo@kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=vda.linux@googlemail.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).