linux-kernel - Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator

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Date:   Wed, 24 Jan 2018 11:43:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator

On 24/01/2018 11:35, Peter Zijlstra wrote:
> On Tue, Jan 23, 2018 at 08:48:13PM +0000, David Woodhouse wrote:
>> On Tue, 2018-01-23 at 21:28 +0100, Borislav Petkov wrote:
>>>
>>>>        flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
>>>> -     asm("push %[flags]; popf; call *%[fastop]"
>>>> -         : "=a"(rc) : [fastop]"r"(fop), [flags]"r"(flags));
>>>> +     asm("push %[flags]; popf; " CALL_NOSPEC
>>>> +         : "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));
>>>
>>> Oh, "thunk_target" is magical.
>>
>> You can use THUNK_TARGET(fop), which will be "rm" on 32-bit and avoids
>> register starvation in some cases (I don't think the hyperv calls
>> worked until I did that).
> 
> The reason I didn't use THUNK_TARGET() was exactly because it used "rm"
> and the current code did "r" only. I'm happy to change if people can
> agree on something ;-)

In practice, "fastop" is going to be in a register because of how it's
computed, but "rm" is okay.

Thanks,

Paolo