linux-kernel - Re: [PATCH 00/19] Refresh queued CET virtualization series

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Date:   Thu, 16 Jun 2022 16:18:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, seanjc@...gle.com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH 00/19] Refresh queued CET virtualization series

On Thu, Jun 16, 2022 at 12:21:20PM +0200, Paolo Bonzini wrote:
> On 6/16/22 12:12, Peter Zijlstra wrote:
> > Do I understand this right in that a host without X86_KERNEL_IBT cannot
> > run a guest with X86_KERNEL_IBT on? That seems unfortunate, since that
> > was exactly what I did while developing the X86_KERNEL_IBT patches.
> > 
> > I'm thinking that if the hardware supports it, KVM should expose it,
> > irrespective of the host kernel using it.
> 
> For IBT in particular, I think all processor state is only loaded and stored
> at vmentry/vmexit (does not need XSAVES), so it should be feasible.

That would be the S_CET stuff, yeah, that's VMCS managed. The U_CET
stuff is all XSAVE though.

But funny thing, CPUID doesn't enumerate {U,S}_CET separately. It *does*
enumerate IBT and SS separately, but for each IBT/SS you have to
implement both U and S.

That was a problem with the first series, which only implemented support
for U_CET while advertising IBT and SS (very much including S_CET), and
still is a problem with this series because S_SS is missing while
advertised.