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Subject: Re: [PATCH 1/3] KVM: x86: use native MSR ops for SPEC_CTRL
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <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
References: <1519249297-73718-1-git-send-email-pbonzini@redhat.com>
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 <20180222170717.GP31483@char.us.oracle.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d66a2358-cfac-f441-3b59-e44674b6a792@redhat.com>
Date:   Fri, 23 Feb 2018 10:37:49 +0100
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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