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Subject: Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator
To: Peter Zijlstra <peterz@infradead.org>,
 David Woodhouse <dwmw2@infradead.org>
Cc: 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>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <9fe90910-7908-f570-0088-568213f52a78@redhat.com>
Date: Wed, 24 Jan 2018 11:43:05 +0100
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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