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Date: Thu, 25 Jan 2018 10:34:53 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>
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>
Subject: Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator
Message-ID: <20180125093453.GX2228@hirez.programming.kicks-ass.net>
References: <20180123152539.374360046@infradead.org>
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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?

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