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Re: [Xen-devel] [PATCH v8 15/17] x86/ctxt: Issue a speculation barrier between vcpu contexts
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
- Date: Mon, 15 Jan 2018 13:54:45 +0100
- Delivery-date: Mon, 15 Jan 2018 12:55:09 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Fri, 2018-01-12 at 18:01 +0000, Andrew Cooper wrote:
>
> @@ -1736,6 +1736,9 @@ void context_switch(struct vcpu *prev, struct
> vcpu *next)
> }
>
> ctxt_switch_levelling(next);
> +
> + if ( opt_ibpb )
> + wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
> }
>
If you're doing that without an 'else lfence' don't you need to include
a comment with your proof that it's safe to do so, and the CPU can't
speculate a taken conditional branch and all the way to a problematic
instruction?
Also... if you're doing that in context_switch() does it do the right
thing with idle? If a CPU switches to the idle domain and then back
again to the same vCPU, does that do the IBPB twice?
For vmx we only really need IBPB when we do VMPTRLD, right? Attachment:
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