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From: Wanpeng Li <kernellwp@gmail.com>
Date: Thu, 3 May 2018 20:01:24 +0800
Message-ID: <CANRm+CzXieWra9uyEuVpHF+5=Vq+JN4QY9BvJXRL3DEwqBGAag@mail.gmail.com>
Subject: Re: [PATCH v6 2/5] KVM: x86: Add IBPB support
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: KarimAllah Ahmed <karahmed@amazon.de>, kvm <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, "the arch/x86 maintainers" <x86@kernel.org>,
	Ashok Raj <ashok.raj@intel.com>, Asit Mallick <asit.k.mallick@intel.com>,
	Dave Hansen <dave.hansen@intel.com>, Arjan Van De Ven <arjan.van.de.ven@intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>, Linus Torvalds <torvalds@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>, Andi Kleen <ak@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>, Dan Williams <dan.j.williams@intel.com>,
	Jun Nakajima <jun.nakajima@intel.com>, Andy Lutomirski <luto@kernel.org>,
	Greg KH <gregkh@linuxfoundation.org>, Peter Zijlstra <peterz@infradead.org>,
	David Woodhouse <dwmw@amazon.co.uk>
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2018-05-03 17:19 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
> On 03/05/2018 03:27, Wanpeng Li wrote:
>> So for 1) guest->guest attacks 2) guest/ring3->host/ring3 attacks 3)
>> guest/ring0->host/ring0 attacks, if IBPB is enough to protect these
>> three scenarios and retpoline is not needed?
>
> In theory yes, in practice if you want to do that IBPB is much more
> expensive than retpolines, because you'd need an IBPB on vmexit or a
> cache flush on vmentry.

https://lkml.org/lkml/2018/1/4/615 Retpoline is not recommended on
Skylake, so we need to pay the penalty for IBPB flush on each vmexit I
think.

Regards,
Wanpeng Li