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Subject: Re: [PATCH v3 0/4] KVM: Expose speculation control feature to guests
To: Paolo Bonzini <pbonzini@redhat.com>, "Raj, Ashok" <ashok.raj@intel.com>
References: <1517271028-15916-1-git-send-email-karahmed@amazon.de>
 <1517302830.18619.78.camel@infradead.org>
 <6b7db789-9cc1-1b4d-9209-8d082e0d8def@redhat.com>
 <20180130234822.GA62797@otc-nc-03>
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Cc: David Woodhouse <dwmw2@infradead.org>,
 KarimAllah Ahmed <karahmed@amazon.de>, kvm@vger.kernel.org,
 linux-kernel@vger.kernel.org, x86@kernel.org, Andi Kleen
 <ak@linux.intel.com>, Andrea Arcangeli <aarcange@redhat.com>,
 Andy Lutomirski <luto@kernel.org>, Arjan van de Ven <arjan@linux.intel.com>,
 Asit Mallick <asit.k.mallick@intel.com>, Borislav Petkov <bp@suse.de>,
 Dan Williams <dan.j.williams@intel.com>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
 Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
 Joerg Roedel <joro@8bytes.org>, Jun Nakajima <jun.nakajima@intel.com>,
 Laura Abbott <labbott@redhat.com>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Masami Hiramatsu <mhiramat@kernel.org>, Peter Zijlstra
 <peterz@infradead.org>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?=
 <rkrcmar@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
 Tim Chen <tim.c.chen@linux.intel.com>, Tom Lendacky <thomas.lendacky@amd.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <e9d538c7-45db-ee9d-ceb4-f1c2569a6d59@intel.com>
Date: Tue, 30 Jan 2018 22:54:37 -0800
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On 01/30/2018 04:16 PM, Paolo Bonzini wrote:
> On 30/01/2018 18:48, Raj, Ashok wrote:
>>> Certainly not every vmexit!  But doing it on every userspace vmexit and
>>> every sched_out would not be *that* bad.
>> Right.. agreed. We discussed the different scenarios that doing IBPB
>> on VMexit would help, and decided its really not required on every exit. 
>>
>> One obvious case is when there is a VMexit and return back to Qemu
>> process (witout a real context switch) do we need that to be 
>> protected from any poisoned BTB from guest?
> If the host is using retpolines, then some kind of barrier is needed.  I
> don't know if the full PRED_CMD barrier is needed, or two IBRS=1/IBRS=0
> writes back-to-back are enough.

I think the spec is pretty clear here: protection is only provided
*while* IBRS=1.  Once it goes back to 0, all bets are off.