Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: selftests/x86/fsgsbase_64 test problem
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Borislav Petkov <bp@alien8.de>, Dan Rue <dan.rue@linaro.org>,
        Shuah Khan <shuah@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20180126153631.ha7yc33fj5uhitjo@xps>
 <CALCETrXVFC=6wPNpL0Dc7pkSv4JSaoEZZcu3Rtw0sg7KJsE5Pg@mail.gmail.com>
 <CALCETrUi7Ub2TbFy3Cvj+j4VXZeYULPY+mgL7OX7bz9L8GO9ew@mail.gmail.com>
 <CALCETrWvkd68wPCxGwmzqgsLTr_59+=L9u8obwt+f+oUQwDY=w@mail.gmail.com>
 <CALCETrW5=K5wJqLRZACmJJ+LF8Nb+Yf3OJ1tTNwqiOCxHu3w1Q@mail.gmail.com>
 <CALCETrWq04AngUHkSv-bqDYXDSVH8N0bhXS_Nh63SpMmndexSA@mail.gmail.com>
 <CALCETrUcJ2dVkSXGhtRvQeYQZZhPca95VhjQB8j6dFN6hzceSQ@mail.gmail.com>
 <46328204-e363-e517-f30c-c8c94ac1442c@zytor.com>
 <CALCETrW6SY8ktqSA-r108WkNf15js4eu0grDds=JVbNtBdSGqA@mail.gmail.com>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <624b9e67-14ee-6882-e55e-f337ec2471d3@zytor.com>
Date:   Mon, 29 Jan 2018 10:12:32 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.4.0
MIME-Version: 1.0
In-Reply-To: <CALCETrW6SY8ktqSA-r108WkNf15js4eu0grDds=JVbNtBdSGqA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 01/29/18 08:37, Andy Lutomirski wrote:
> 
> That's what I thought, too, and the SDM does say that, but the SDM
> says all kinds of not-quite-correct things about segmentation.
> 
>> It is pretty much scratch space (I have
>> suggested using it for the gsbase once all those issues get sorted out,
>> because it lets the paranoid code do something like:
>>
>>         rdgsbase %rax
>>         push %rax       /* Save old gsbase */
>>         push %rax       /* Reserve space on stack */
>>         sgdt -2(%rsp)   /* We don't care about the limit */
>>         pop %rax        /* %rax <- gdtbase */
>>         mov (%rax),%rax /* GDT[0] holds the gsbase for this cpu */
>>         wrgsbase %rax
> 
> That will utterly suck on non-UMIP machines that have
> hypervisor-provided UMIP emulation.
> 

Is that a valid thing to optimize for, especially given that paranoid
entries aren't the most common anyway?

	-hpa