Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of
 trampoline stack
To:     David Laight <David.Laight@ACULAB.COM>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Joerg Roedel <jroedel@suse.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        "jan.setjeeilers@oracle.com" <jan.setjeeilers@oracle.com>,
        Junaid Shahid <junaids@google.com>,
        "oweisse@google.com" <oweisse@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Alexander Graf <graf@amazon.de>,
        "mgross@linux.intel.com" <mgross@linux.intel.com>,
        "kuzuno@gmail.com" <kuzuno@gmail.com>
References: <20201116144757.1920077-1-alexandre.chartre@oracle.com>
 <20201116144757.1920077-13-alexandre.chartre@oracle.com>
 <CALCETrUSCwtR41CCo_cAQf_BwG7istH6fM=bxWh_VfOjSNFmSw@mail.gmail.com>
 <bc8a254e-deaa-388e-99ea-0291f5625b5b@oracle.com>
 <985e90259d654d7c9eb74398c5d927ee@AcuMS.aculab.com>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <e6e168db-5d16-66a6-e817-6d23ea0bf1eb@oracle.com>
Date:   Tue, 17 Nov 2020 09:27:11 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.12.0
MIME-Version: 1.0
In-Reply-To: <985e90259d654d7c9eb74398c5d927ee@AcuMS.aculab.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 11/16/20 10:24 PM, David Laight wrote:
> From: Alexandre Chartre
>> Sent: 16 November 2020 18:10
>>
>> On 11/16/20 5:57 PM, Andy Lutomirski wrote:
>>> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
>>> <alexandre.chartre@oracle.com> wrote:
>>>>
>>>> When entering the kernel from userland, use the per-task PTI stack
>>>> instead of the per-cpu trampoline stack. Like the trampoline stack,
>>>> the PTI stack is mapped both in the kernel and in the user page-table.
>>>> Using a per-task stack which is mapped into the kernel and the user
>>>> page-table instead of a per-cpu stack will allow executing more code
>>>> before switching to the kernel stack and to the kernel page-table.
>>>
>>> Why?
>>
>> When executing more code in the kernel, we are likely to reach a point
>> where we need to sleep while we are using the user page-table, so we need
>> to be using a per-thread stack.
> 
> Isn't that going to allocate a lot more kernel memory?

That's one of my concern, hence this RFC. The current code is doubling the
task stack (this was an easy solution), so that's +8KB per task. See my
reply to Boris, it has a bit more details.

alex.


> ISTR some thoughts about using dynamically allocated kernel
> stacks when (at least some) wakeups are done by directly
> restarting the system call - so that the sleeping thread
> doesn't even need a kernel stack.
> (I can't remember if that was linux or one of the BSDs)
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>