linux-kernel - Re: selftests/x86/fsgsbase

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Message-ID: <624b9e67-14ee-6882-e55e-f337ec2471d3@zytor.com>
Date:   Mon, 29 Jan 2018 10:12:32 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, Dan Rue <dan.rue@...aro.org>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: selftests/x86/fsgsbase_64 test problem

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