Re: [PATCH 1/3] KVM: x86: use native MSR ops for SPEC_CTRL

[Paolo Bonzini]

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