Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 23 Feb 2018 12:55:43 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Jim Mattson <jmattson@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, stable@vger.kernel.org
Subject: Re: [PATCH 1/3] KVM: x86: use native MSR ops for SPEC_CTRL
Message-ID: <20180223175543.GN8252@char.us.oracle.com>
References: <1519249297-73718-1-git-send-email-pbonzini@redhat.com>
 <1519249297-73718-2-git-send-email-pbonzini@redhat.com>
 <20180222170717.GP31483@char.us.oracle.com>
 <d66a2358-cfac-f441-3b59-e44674b6a792@redhat.com>
 <20180223172220.GK8252@char.us.oracle.com>
 <7534bcd3-e21b-240c-9ed2-62026ba10301@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <7534bcd3-e21b-240c-9ed2-62026ba10301@redhat.com>
User-Agent: Mutt/1.8.3 (2017-05-23)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Feb 23, 2018 at 06:35:30PM +0100, Paolo Bonzini wrote:
> On 23/02/2018 18:22, Konrad Rzeszutek Wilk wrote:
> > On Fri, Feb 23, 2018 at 10:37:49AM +0100, Paolo Bonzini wrote:
> >> On 22/02/2018 18:07, Konrad Rzeszutek Wilk wrote:
> >>>> Having a paravirt indirect call in the IBRS restore path is not a
> >>>> good idea, since we are trying to protect from speculative execution
> >>>> of bogus indirect branch targets.  It is also slower, so use
> >>>> native_wrmsrl on the vmentry path too.
> >>> But it gets replaced during patching. As in once the machine boots
> >>> the assembler changes from:
> >>>
> >>> 	callq 	*0xfffflbah
> >>>
> >>> to
> >>> 	wrmsr
> >>>
> >>> ? I don't think you need this patch.
> >>
> >> Why not be explicit?  According to the spec, PRED_CMD and SPEC_CTRL
> > 
> > Explicit is fine.
> > 
> > But I would recommend you change the commit message to say so, and
> > perhaps remove 'It is also slower' - as that is incorrect.
> 
> Actually it is faster---that's why I made the change in the first place,
> though later I noticed
> 
> > If it is detected to be Xen PV, then yes
> > it will be a call to a function. But that won't help as Xen PV runs in
> > ring 3, so it has a whole bunch of other issues.
> 
> Ok, I wasn't sure about PVH (which runs in ring 0 afair).

Right. PVH is HVM without any emulated devices or BIOSes or such.

In the context of the paravirt ops, Xen PVH == Xen HVM.

Xen PV (and lguests) are the only ones that patch the the
	callq	*0xffffblah

to
	callq	0xffff800

While everyone else does the wrmsr.
> 
> Paolo