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Subject: Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw2@infradead.org>, Borislav Petkov <bp@alien8.de>,
 Thomas Gleixner <tglx@linutronix.de>, Josh Poimboeuf <jpoimboe@redhat.com>,
 linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>,
 Ashok Raj <ashok.raj@intel.com>, Tim Chen <tim.c.chen@linux.intel.com>,
 Andy Lutomirski <luto@kernel.org>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Greg KH <gregkh@linuxfoundation.org>, Andrea Arcangeli
 <aarcange@redhat.com>, Andi Kleen <ak@linux.intel.com>,
 Arjan Van De Ven <arjan.van.de.ven@intel.com>,
 Dan Williams <dan.j.williams@intel.com>,
 Jun Nakajima <jun.nakajima@intel.com>,
 Asit Mallick <asit.k.mallick@intel.com>, Jason Baron <jbaron@akamai.com>
References: <20180123152539.374360046@infradead.org>
 <20180123152638.336674335@infradead.org>
 <20180123202859.qc5frxyonavtkhpz@pd.tnic>
 <1516740493.13558.3.camel@infradead.org>
 <20180124103525.GR2228@hirez.programming.kicks-ass.net>
 <9fe90910-7908-f570-0088-568213f52a78@redhat.com>
 <20180125093453.GX2228@hirez.programming.kicks-ass.net>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <df0ca3ce-3185-d4d7-5467-bee3bbbf7cce@redhat.com>
Date: Fri, 26 Jan 2018 11:57:00 +0100
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On 25/01/2018 10:34, Peter Zijlstra wrote:
> On Wed, Jan 24, 2018 at 11:43:05AM +0100, Paolo Bonzini wrote:
>> 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.
> 
> OK, so the other occurence in that file uses "+S", which is the SI
> register. That cannot use THUNK_TARGET(), right?

Nope, it reads the output in %esi too.

> So do you want one THUNK_TARGET and one open coded, or keep the patch as
> is (both open coded) ?

Open coded is okay.

Paolo