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Re: [PATCH] x86/cpuid: prevent decreasing of hypervisor max leaf on migration
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 2 Nov 2021 13:32:45 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 02 Nov 2021 12:33:11 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Nov 02, 2021 at 01:00:07PM +0100, Jan Beulich wrote:
> On 27.10.2021 16:00, Roger Pau Monne wrote:
> > In order to be compatible with previous Xen versions, and not change
> > max hypervisor leaf as a result of a migration, keep the clamping of
> > the maximum leaf value provided to XEN_CPUID_MAX_NUM_LEAVES, instead
> > of doing it based on the domain type. Also set the default maximum
> > leaf without taking the domain type into account. The maximum
> > hypervisor leaf is not migrated, so we need the default to not regress
> > beyond what might already be reported to a guest by existing Xen
> > versions.
>
> While this is the missing description to the patch I had submitted
> back in May upon Andrew's request, I have to admit that I don't
> consider this a satisfactory explanation. Shouldn't hypervisor
> leaves undergo similar leveling as other leaves? I.e. upon
> migration leaves or individual bits should neither disappear nor
> appear?
Indeed, but hypervisor max leaves is not properly migrated, as
hv{,2}_limit is not set unless explicitly passed by the toolstack.
> I continue to consider it at least suspicious that HVM guests get
> 5 leaves reported when only 4 are really meaningful to them.
That's indeed fine to fix, but we would need to start saving/restoring
hypervisor max leaf unconditionally as part of the cpuid policy (ie:
set hv{,2}_limit at domain create to the default limit), and populate
it with 5 for backwards compatibility on restore if not set.
> I
> see this has gone in, so I'm likely to trip up on this again in
> the future. Might result in the same patch again then if I don't
> end up doing archeology at that point ...
Maybe there's some comment we could add in the code to make this
clearer? I didn't realize either when reviewing as didn't pay close
attention to how hv{,2}_limit is used.
I hope the above comments help clarify why the current behavior was
an issue.
Regards, Roger.
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