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Re: [Xen-devel] [PATCH 0/9] xen/amd: Support for guest MSR_VIRT_SPEC_CTRL support
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 4 Dec 2018 11:26:31 +0000
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 04 Dec 2018 11:26:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 04/12/2018 09:45, Jan Beulich wrote:
>>>> On 03.12.18 at 17:31, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 03/12/2018 16:24, Jan Beulich wrote:
>>>>>> On 03.12.18 at 17:18, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> This is a lingering TODO item from XSA-263. It adds support AMD's
>>>> MSR_VIRT_SPEC_CTRL interface, and changes Xen's "boot time global" SSBD
>>>> setting into a per-vcpu setting.
>>>>
>>>> This can be found on:
>>>> git://xenbits.xen.org/people/andrewcoop/xen.git xen-virt-spec-ctrl-v1
>>>>
>>>> The start of the series is some cleanup. It then teaches Xen to recognise
>> the
>>>> available interfaces (including MSR_VIRT_SPEC_CTRL from a hypervisor), then
>>>> how to safely context switch the per-core LS_CFG on Fam17h, an finally to
>>>> expose support to guests.
>>>>
>>>> I've got some further MSR work coming because we have to fix the
>>>> default-leakiness of MSRs in this range, because a guest becomes unsafe to
>>>> migrate as soon as it reads any of the pipeline control MSRs.
>>> I've seen you mention this elsewhere, but I'm still unclear about
>>> the "why" part here.
>> Because the existence (or not) are model specific, the details read are
>> non-architectural, not always the same on minor variations of the same
>> platform, and definitely not the same across different models.
> But migration then is only one (albeit perhaps the major) aspect.
> CPUID overrides altering e.g. the model number would similarly
> be a problem if we were to imply from reads of these MSRs that
> "problematic" conclusions would be drawn by the guest.
>
> Otoh I can't see how migration to an identical CPU model system
> would become unsafe.
Identical system with identical firmware and configuration - yes we'd
expect them to have identical values in these configuration.
>
> Nor can I see how hiding these MSRs from guests would improve
> the situation in this regard: Guests may still draw unwanted
> conclusions from not being able to read these MSRs, or reading
> all zeros.
I can't help but feel that the observations you've made answer the
question very succinctly.
Of course we can't prevent the guest drawing conclusions from the
absense/presence of the information. What we can (and must) ensure is
that the information that is available (i.e. a #GP fault) does not have
any details which are specific to the processor that the VM happened to
boot on.
~Andrew
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