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From: | David Hildenbrand |
Subject: | Re: [PATCH 0/7] i386: Add `machine` parameter to query-cpu-definitions |
Date: | Fri, 25 Oct 2019 16:23:59 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 |
For example -machine s390-virtio-ccw-3.1 -cpu z14 will not have the multiple epoch facility and -machine s390-virtio-ccw-4.0 -cpu z14 will have the multiple epoch facility. As migration does always require the tuple of machine and cpu this is save. I fail to see what the benefit of an explicit z14-3.1 would be.AFAIKS the only real benefit of versioned CPU models is that you can add new CPU model versions without new QEMU version.This is very important for backporting CPU security fixes to existing QEMU releases.
I'd say it's not really relevant for backporting per se. It's relevant for automatically enabling security fixes when not using the host model. That part I understand. Less likely to make mistakes when explicitly specifying CPU models.
I once was told that if a user actually specified an explicit CPU model in the libvirt XML ("haswell-whatever"), you should not go ahead and make any later changes to that model (guest ABI should not change when you update/restart the guest ...). So this only applies when creating new guests? Or will you change existing model definitions implicitly?
Then you can specify "-cpu z13-vX" or "-cpu z13 -cpuv X" (no idea how versioned CPU model were implemented) on any QEMU machine. Which is the same as telling your customer "please use z13,featX=on" in case you have a good reason to not use the host model (along with baselining) but use an explicit model. If you can change the default model of QEMU machines, you can automate this process. I am pretty sure this is a corner case, though (e.g., IBRS). Usually you have a new QEMU machine and can simply enable the new feature from that point on.There are now 4 Haswell variants, only some of which are runnable on any given host, depending on what microcode the user has installed or what particular Haswell silicon SKU the user purchased. Given the frequency of new CPU flaws arrived since the first Meltdown/Spectre, this isn't a corner case, at least for the x86 world & Intel in particular. Other arches/vendors haven't been quite so badly affected in this way.
On s390x you can assume that such firmware/microcode updates will be on any machine after some time. That is a big difference to x86-64 AFAIK.
If we tied each new Haswell variant to a machine type, then users would be blocked from consuming a new machine type depending on runnability of the CPU model. This is not at all desirable, as mgmt apps now have complex rules on what machine type they can use.
So you actually want different CPU variants, which you have already, just in a different form. (e.g., "haswell" will be mapped to "haswell-whatever", just differently via versions)
When dealing with backporting patches for new CPU hardware flaws, the new CPU features are backported to many old QEMU versions. The new machine types are not backportable.
That part I understand.
Both these called for making CPU versioning independant of machine type versioning. Essentially the goal with CPU versioning is that the user can request a bare "Haswell" and libvirt (or the mgmt app) will automatically expand this to the best Haswell version that the host is able to support with its CPUs / microcode / BIOS config combination.
So if I do a "-cpu haswell -M whatever-machine", as far as I understood reading this, I get the "default CPU model alias for that QEMU machine" and *not* the "best Haswell version that the host is able to support".
Or does the default actually also depend on the current host? -- Thanks, David / dhildenb
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