By reading that advisory and information posted here (
https://blog.xenproject.org/2018/01/04/xen-project-spectremeltdown-faq/),
it seems that there are 2 possible short-term mitigations against
Meltdown for QubesOS 3.2 users.
-  Move PV VMs to HVM.
-  Move PV VMs to use 32-bit kernels. It should prevent to use
Meltdown/SP3 against the hypervisor, but it won't prevent it against the
kernel itself.  Then update when newer 32-bit kernel with KPIT are
available.

Qubes 4 users shouldn't be affected by SP3/Meltdown, but should be
affected from SP1/SP2/Spectre.


On 01/04/2018 10:53 PM, Chris Drake wrote:
> It is very clear: https://xenbits.xen.org/xsa/advisory-254.html
>
> IMPACT
> ======
>
> Xen guests may be able to infer the contents of arbitrary host memory,
> including memory assigned to other guests.
>
> VULNERABLE SYSTEMS
> ==================
>
> Systems running all versions of Xen are affected.
>
> MITIGATION
> ==========
>
> There is no mitigation for SP1 and SP2.
>
> RESOLUTION
> ==========
>
> There is no available resolution for SP1 or SP3.
>
>
> For those unaware - this is a hardware fault.  CPUs make use of speculative 
> execution (Spectre) or Pipelines (Meltdown) - both of which can be used to 
> attempt to access illegal memory.  The access fails, however, it's possible 
> to use the "stolen" memory before the access-fail is enforced in a way that 
> makes it available on a side-channel (cache in these exploits, but could be 
> anything else like ports/dma) to any non-privileged process.
>

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