Message-ID: <553AB960.6090705@zytor.com>
Date: Fri, 24 Apr 2015 14:45:04 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>,
        Denys Vlasenko <vda.linux@googlemail.com>
CC: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute
 issue
References: <5d120f358612d73fc909f5bfa47e7bd082db0af0.1429841474.git.luto@kernel.org> <CALCETrU1X4=GHmCcBfoHYDqUKAB-ybhgYpEHEGQVK+0_wwCJBA@mail.gmail.com> <CAK1hOcP0_cMbrhYnwapQE5wK5NTJUXbK64rq7xkSq-HEBXmdDQ@mail.gmail.com> <CALCETrVjcw9TAKm+GGzJtzUUPpovJYZMn4vXc=cYG+s5B57h5Q@mail.gmail.com>
In-Reply-To: <CALCETrVjcw9TAKm+GGzJtzUUPpovJYZMn4vXc=cYG+s5B57h5Q@mail.gmail.com>
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On 04/24/2015 01:50 PM, Andy Lutomirski wrote:
>>
>> Exactly my feeling. What are you trying to save? About four CPU
>> cycles of checking %ss != __KERNEL_DS on each switch_to?
>> That's not worth bothering about. Your last patch seems to be perfect.
> 
> We'll have to do the write to ss almost every time an AMD CPU sleeps
> in a syscall.  Maybe that's still not a big deal.
> 

Once you're sleeping anyway, I don't think so.

	-hpa


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