linux-kernel - Re: [PATCH 1/3] KVM: x86: use native MSR ops for SPEC

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Message-ID: <20180223172220.GK8252@char.us.oracle.com>
Date:   Fri, 23 Feb 2018 12:22:20 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: x86: use native MSR ops for SPEC_CTRL

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.

> should be passed down to the guest without interception so it's safe to
> do this.  On the other hand, especially with nested virtualization, I
> don't think you can absolutely guarantee that the paravirt call will be
> patched to rdmsr/wrmsr.

<scratches his head> 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.

If it detects it as KVM or Xen HVM guest it will patch it with the default
- which is normal MSRs. Ditto for HyperV.

But <shrugs> no biggie - explicit is fine, just nagging on the commit
message could use a bit of expansion.

> Paolo
>