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Re: [Xen-devel] (partial) Spectre v2 mitigation without on Skylake IBRS

On 26/02/18 11:49, Jan Beulich wrote:
>>>> On 26.02.18 at 11:18, <jgross@xxxxxxxx> wrote:
>> On 26/02/18 10:44, Jan Beulich wrote:
>>> if running PV Linux on older Xen (4.5 and earlier) is relevant, it may be
>>> necessary to use a mechanism other than IBRS to mitigate Spectre v2
>>> on Skylake. That is because the new MSR value can't be migrated
>>> prior to migration v2. Of course one option would be to retrofit some
>>> mechanism into newer Xen versions that makes them accept whatever
>>> extension to e.g. struct hvm_hw_cpu one might want to invent for
>>> the older Xen versions. But that doesn't seem very desirable.
>> Can you please elaborate a little bit more what the real problem is?
>> I _think_ you are referring to the problem that a pv kernel would want
>> to use IBRS for mitigation of Spectre V2 and after a migration that
>> setting would be lost.
> "Lost" is the wrong term imo: A hypervisor that's been patched for
> Spectre v2 (and that's a prereq anyway, because we want the
> kernel to use IBPB, which utilizes an MSR that doesn't need
> migrating) should at least do _something_ with the MSR (when it's
> non-zero). The most natural thing (imo) is to make those older
> hypervisors support XEN_DOMCTL_{get,set}_vcpu_msrs. That in
> turn calls for the tool stack to gain the check that Andrew had
> added to libxc in db24f7f012 ("libxc: use an explicit check for PV
> MSRs in xc_domain_save() "), causing migration to fail when the
> MSR is non-zero on any of the guest's vCPU-s.
>> If this is the case I believe the easiest solution would be to let the
>> kernel set the MSR again after leaving suspended state. suspend/resume
>> require hooks in pv kernels after all.
> Hmm, this could be leveraged irrespective of what I've written
> above - the kernel could then also clear the MSR during suspend,
> thus allowing the check in libxc to pass.

Something like the attached patch?


Attachment: 0001-x86-xen-zero-MSR_IA32_SPEC_CTRL-before-suspend.patch
Description: Text Data

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