linux-kernel - Re: [PATCH v2 1/7] KVM:VMX: Define CET VMCS fields and bits

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Date:   Tue, 29 Jan 2019 16:29:17 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     rkrcmar@...hat.com, sean.j.christopherson@...el.com,
        jmattson@...gle.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, mst@...hat.com, yu-cheng.yu@...el.com,
        yi.z.zhang@...el.com, hjl.tools@...il.com,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v2 1/7] KVM:VMX: Define CET VMCS fields and bits

On Tue, Jan 29, 2019 at 04:19:34PM +0100, Paolo Bonzini wrote:
> On 28/01/19 11:33, Yang Weijiang wrote:
> >> There is no code in this series to pass these fields to and from
> >> userspace, and also to save/restore U_CET, INT_SSP_TAB, PL0_SSP and
> >> PL3_SSP across context switches.
> >>
> > The kernel consumes these MSRs, please see kernel CET patch:
> > https://lkml.org/lkml/fancy/2018/11/20/225
> 
> Still, even if the kernel saves these fields across context switch in
> XSAVE areas, KVM must support accesses to the MSRs from userspace, for
> example in order to perform live migration.
> 
> For example, when reading/writing these in kvm_set_msr or
> kvm_get_msr_common, you would have to do a read/write from the host
> MSRs.  You also have to put kvm_load_guest_fpu/kvm_put_guest_fpu calls
> in __msr_io.
Thanks, you're right, if we want to support live migration we need do that.
However, this feature relies on quite a few external
dependencies, e.g., cpu capabilities, kernel version and configuration, we
don't support live migration right now. what's your opinion?
> 
> Thanks,
> 
> Paolo
> 
> >> In addition, PL1_SSP and PL2_SSP should be supported even if the guest
> >> doesn't use them.  It makes sense to avoid intercepting them, but they
> >> should still be supported and switched (possibly only if nonzero).
> >>
> >> Am I missing something, for example a dependency on host CET support?
> >> If not, how was this series tested?
> >>
> > The guest CET feature is tested with kernel CET patches on internal
> > virtual platform.
> >