linux-kernel - Re: [PATCH] x86/entry/64/compat: Preserve r8-r11 in int $0x80

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Date:   Wed, 18 Apr 2018 19:13:04 +0200
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH] x86/entry/64/compat: Preserve r8-r11 in int $0x80

On 04/18/2018 06:53 PM, Andy Lutomirski wrote:
> On Tue, Apr 17, 2018 at 8:00 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> This means that the new behavior is there for some 8 years already.
>> Whoever was impacted by it, probably already switched to the new ABI.
>>
>> Current ABI is "weaker", it allows kernel to save fewer registers.
>>
>> Which is generally a good thing, since saving/restoring things cost
>> cycles, and sometimes painful on entry paths where you may desperately
>> need a scratch register or two. (Recall this one? -
>> ...
>>          movq    %rsp, PER_CPU_VAR(rsp_scratch)
>>          movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
>>          /* Construct struct pt_regs on stack */
>>          pushq   $__USER_DS                      /* pt_regs->ss */
>>          pushq   PER_CPU_VAR(rsp_scratch)        /* pt_regs->sp */
>> ...
>> wouldn't it be _great_ if one of GPRs would be available here
>> to hold userspace %rsp?
>> )
> 
> But this is the int $0x80 entry, which uses the stack sanely and
> doesn't have this problem at all.

It was a general point why not committing to save every register
may help on the callee (in this case kernel) side.