From: Borislav Petkov <bp@suse.de>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH urgent] x86, asm: Disable opportunistic SYSRET if regs->flags has TF set
Date: Thu, 2 Apr 2015 08:16:53 +0200 [thread overview]
Message-ID: <20150402061653.GA4579@pd.tnic> (raw)
In-Reply-To: <551C6098.9030705@redhat.com>
On Wed, Apr 01, 2015 at 11:18:16PM +0200, Denys Vlasenko wrote:
> On 04/01/2015 09:25 PM, Andy Lutomirski wrote:
> > Fix it by using IRET to restore TF. Since it's late, I'm keeping
> > this minimal and keeping "testq" instead of switching to "testl".
>
> Changing to "testl" here wins nothing.
Except less data (a dword) being shuffled and tracked for dependencies
in the machine instead of qword.
> Since r11 is used, REX prefix will be encoded anyway.
As a future cleanup, one could use one of the "old", i.e. not-extended
registers to save 2 bytes per insn (REX pfx and ModRM) but one has to
remember to do
mov %rax, %r11
in the end.
And yep, it should preferrably be %rax as we have opcode 0xa9 which
tests an immediate and RAX and saves us the ModRM as we don't need to
specify a register.
orig:
a42: 49 f7 c3 00 00 01 00 test $0x10000,%r11
a49: 75 41 jne a8c <opportunistic_sysret_failed>
Andy:
a42: 49 f7 c3 00 01 01 00 test $0x10100,%r11
a49: 75 41 jne a8c <opportunistic_sysret_failed>
testl:
a42: 41 f7 c3 00 01 01 00 test $0x10100,%r11d
a49: 75 41 jne a8c <opportunistic_sysret_failed>
%rax:
a42: a9 00 01 01 00 test $0x10100,%eax
a47: 75 41 jne a8a <opportunistic_sysret_failed>
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
--
prev parent reply other threads:[~2015-04-02 6:18 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-01 19:25 [PATCH urgent] x86, asm: Disable opportunistic SYSRET if regs->flags has TF set Andy Lutomirski
2015-04-01 21:18 ` Denys Vlasenko
2015-04-02 6:16 ` Borislav Petkov [this message]
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