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

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Date:   Fri, 23 Feb 2018 10:37:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.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 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
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.

Paolo